How to Check Cross Domain Tracking in Google Analytics

Verifying your cross-domain tracking in Google Analytics is the single most important step after you set it up. Without a proper check, you could be basing marketing decisions on fragmented data, missing out on tracking user journeys across different websites you own. This guide will show you how to confirm everything is working correctly so you can trust your data and accurately measure performance.

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What Is Cross-Domain Tracking and Why Is It So Important?

Cross-domain tracking, often called site linking, allows Google Analytics to see a user's journey across two or more related, but separately owned, domains as a single, continuous session.

Imagine a user clicks a Facebook ad for your e-commerce store (mystore.com). They browse a few products and then click "Checkout." Your checkout process is handled by a third-party shopping cart platform, like Shopify or another provider (shop.provider.com).

Without cross-domain tracking, Google Analytics registers this as two separate events:

• User 1: Arrives at mystore.com from Facebook, looks at products, and then "leaves" the site.
• User 2: A brand new user mysteriously appears at shop.provider.com and makes a purchase.

This broken journey creates massive data problems:

• Inflated User Counts: A single person is now counted as two, skewing your user and session metrics.
• Lost Attribution: The purchase on shop.provider.com is incorrectly attributed to "Referral" traffic from mystore.com, completely erasing the fact that the Facebook ad was the original source of the customer.
• Inaccurate Funnel Analysis: You can't see the full path from initial visit to final conversion, making it impossible to optimize your user experience.

When working correctly, cross-domain tracking stitches this journey together. It tells Google Analytics that the person who landed on mystore.com is the exact same person who continued to shop.provider.com. This preserves the original traffic source (Facebook) and gives you clean, accurate attribution data.

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How it Works: The Magic of the Linker Parameter

Cross-domain tracking works by passing a unique identifier, called the Client ID, between your domains. When a user clicks a link from your first domain (Domain A) to your second (Domain B), Google's linker script appends a special parameter to the URL.

It looks something like this:

https://www.domain-b.com/?_gl=1*1ykrm8j*_ga*MTExNzI3MjUzMS4xNzAzNDA...

That _gl=... parameter contains the Client ID and session information from Domain A. When the Google Analytics tag fires on Domain B, it sees this parameter in the URL, grabs the Client ID, and continues the session instead of starting a new one. Understanding this key piece of the puzzle is critical for knowing how to check if it's working.

5 Methods to Verify Your Cross-Domain Tracking Setup

These five methods range from a simple visual check to a deep dive into your analytics reports. Using a combination of them will give you complete confidence in your setup.

Method 1: Manually Check the URL for the Linker Parameter

This is the fastest and most direct way to see if the first part of your cross-domain setup is functioning. It simply involves clicking a link and looking at the address bar.

Step-by-Step Instructions:

1. Open a new browser window (ideally in Incognito or Private mode to avoid being influenced by existing cookies).
2. Go to your primary domain (e.g., mystore.com).
3. Navigate to and click on any link that is supposed to take you to your secondary domain (e.g., your "Checkout" or "Book Now" button).
4. Once the second domain's page loads, immediately look at the URL in your browser's address bar.

What to Look For: The URL of the second domain should have a _gl parameter appended to it. If you see it, that's a great sign! It means the script on your first domain is correctly "decorating" the outbound links.

If you do NOT see the _gl parameter, the problem lies with the setup on your primary domain. Check your Google Tag Manager (GTM) or gtag.js configuration to ensure that the cross-domain settings are applied correctly.

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Method 2: Use Google Analytics Real-Time Reports

This method lets you watch the session transition in real-time within Google Analytics itself, giving you immediate confirmation that GA is interpreting the user journey correctly.

Step-by-Step Instructions:

1. Open two browser windows side-by-side. In one, navigate to your primary website. In the other, open Google Analytics and go to Reports > Real-time.
2. On your website, to make your session easy to identify, add a unique UTM parameter to the URL and press Enter. For example: https://mystore.com?utm_source=test&utm_medium=cd_verification
3. In the Real-time report, look for your session to appear under the 'Session source / medium' card. You should see "test / cd_verification." The count of "Users in Last 30 Minutes" should be 1.
4. Now, back on your website window, click the link to cross over to your secondary domain (shop.provider.com).
5. Watch the Real-time report closely. The "Active Page" should update to show the page on your second domain, but the overall User count should remain at 1 and your traffic source should stay as "test / cd_verification."

The Fail Condition: If you see the user count briefly jump to 2, and a new session appears with the source listed as your primary domain (e.g., "mystore.com / referral"), your tracking is broken. This means GA failed to stitch the sessions together.

Method 3: Confirm Your 'Unwanted Referrals' List

While this is part of the setup, it's a critical item to check for verification. The Unwanted Referrals list (formerly Referral Exclusion List) tells Google Analytics, "If traffic shows up from these domains, don't start a new session. It's really part of our own user's journey."

Step-by-Step Verification in GA4:

1. Navigate to your Google Analytics Admin panel (the gear icon in the bottom-left).
2. In the Property column, click on Data Streams and select your web stream.
3. Click on Configure tag settings at the bottom.
4. Under the Settings section, click Show more if needed, and then select List unwanted referrals.
5. Verify that BOTH of your domains (and any others involved) are listed here. For our example, both mystore.com and shop.provider.com should be on the list.

If any of your domains are missing, users navigating between them will be incorrectly categorized as "Referral" traffic, which is a key symptom of broken cross-domain tracking.

Method 4: Use Google Tag Manager's Preview Mode

If you're using Google Tag Manager to deploy your GA tags, its built-in Preview Mode is one of the most powerful tools for debugging. It allows you to check if the same Client ID is being used across both domains.

Step-by-Step Instructions:

1. In your GTM workspace, click the Preview button in the top right.
2. Enter the URL of your primary domain and click Connect. Your site will open in a new tab with the debug panel connected.
3. In the Tag Assistant (preview) window or tab, click on an event like Container Loaded or Initialization on the left-hand timeline.
4. Select your GA4 Configuration Tag. Navigate to the Variables tab or look for the request details and find a key named &cid or "Client ID". Record its value — it will be a long string of numbers (e.g., 1117272531.170340...). This is your unique ID for this session.
5. Go back to your website tab and click the link to cross to your secondary domain. Keep the Tag Assistant window open.
6. The Tag Assistant will now load the data for the second domain. Look at the Initialization or Page View event on the second domain, select the GA4 tag, and inspect its Client ID again.

The Success Condition: The Client ID on the second domain must be identical to the Client ID from the first domain. If they match, your setup is working perfectly. If they are different, GA is generating a new ID, and the session is being broken.

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Method 5: Check Your Reports for Self-Referrals

This final method is like an audit of your historical data. If cross-domain tracking has been broken for a while, the evidence will be visible in your standard acquisition reports.

Step-by-Step Instructions:

1. In GA4, go to Reports > Acquisition > Traffic acquisition.
2. In the table, look at the Session source / medium column. You can also change the primary dimension to just Session source for a cleaner view.
3. Scan the list of sources for any of your own domains. Do you see mystore.com listed as a source of traffic driving conversions?

If you see your own domain acting as a referral source, it's a definitive sign that your cross-domain tracking is — or has recently been — incorrectly configured. That "referral" traffic is actually just users moving from one part of your site to another, and their original traffic source (Paid Social, Organic Search, etc.) is being lost.

Final Thoughts

Ensuring your cross-domain tracking works correctly is fundamental to having data you can rely on. By following these validation methods — from checking the URL linker to auditing your Real-time and Acquisition reports — you can confidently track the entire user journey, properly attribute your marketing efforts, and make smarter, data-driven decisions.

Manually tracking down parameters and hopping between reports to stitch together a narrative is often the most frustrating part of data analysis. At Graphed, we eliminate this friction by connecting directly to all your data sources — like Google Analytics, Shopify, and Facebook Ads — in one click. You can simply ask for the outcome you want, such as "Show me a dashboard of a user's journey from their first ad click to their final purchase," and we build you a real-time, unified view without you needing to play detective. The dashboards pull in live data, so you always see a clean, holistic picture of what's truly driving growth for your business. Interested? Create a free dashboard by connecting your data sources with Graphed today.