Ad Tracking Diagnostic: a free GCLID and UTM form field checker
Paste a URL and the tool checks whether Google Ads click IDs and UTM values are captured, kept and written into hidden form fields. It then generates a Google Tag Manager script for the form platform it finds.
- Tool
- Ad Tracking Diagnostic
- License
- MIT
- Built
- Mar 2026
- Stack
- Static HTML and JavaScript, GitHub Pages, GTM Custom HTML
- Cost
- Free, no login
The idea
Offline conversion imports fail quietly when the GCLID or UTM values never reach the hidden fields on a lead form. The CRM record looks fine, but Google Ads never gets credit for the sale.
One common case: Google's standard script uses getElementById(), which fails on Marketo forms because the hidden fields have a name attribute and no id.
How we built it, step by step
- Built a paste-a-URL scannerThe scanner runs 6 checks: GCLID, all 5 UTM parameters, localStorage persistence, hidden form fields, platform detection and script generation.
- Detected the form platformIt recognizes Marketo, HubSpot, Gravity Forms, Shopify, GTM and GA4, then writes a GTM Custom HTML script for what it found.
- Wrote 5 platform scriptsReady scripts cover Salesforce and Marketo, HubSpot, WordPress forms (Gravity Forms, CF7, WPForms, Elementor, Ninja Forms), Shopify and a universal version.
- Fixed the Marketo field problemThe Marketo script finds hidden fields with querySelector('[name=...]') instead of getElementById(), so values land in the right fields.
- Added a bookmarkletThe bookmarklet opens a diagnostic panel on any live form page and generates a script that uses that page's real field names.
- Added click simulatorsSimulators fake a Google Ads click with a GCLID, a full UTM plus GCLID visit, or every platform namespace, so you can test without spending on clicks.
- Documented the setupThe wiki covers GTM deployment, setup per platform, troubleshooting and console debugging.
What it does
- GCLID and UTM capture checks
- localStorage persistence check
- Hidden form field detection
- Generated GTM Custom HTML script
- Bookmarklet for live form pages
- Click and UTM simulators
- Reference table of offline import limits
How people find it
- Hosted free on GitHub Pages, single index.html
- MIT-licensed source on GitHub
- Wiki guides for GTM, HubSpot, Marketo, WordPress and Shopify
- Deploys through Google Tag Manager Custom HTML
Questions people ask about this work
How do I check if GCLID is passed to HubSpot?
Open your landing page with a test GCLID in the URL, then check whether the value is stored and written into the hidden field on the HubSpot form. The diagnostic runs that check from a pasted URL and can simulate the ad click for you. If the field stays empty, it generates a HubSpot script to deploy in GTM.
Why is my GCLID hidden field empty on a Marketo form?
Marketo hidden fields usually have a name attribute and no id, so scripts that use getElementById() can't find them. Selecting the field with querySelector('[name=...]') fixes it. Our Marketo script does this.
How long do I have to import offline conversions into Google Ads?
The tool's reference table lists a 90-day window from the click, a 4 to 6 hour delay on the first import and a 14-day initial lookback. Capture the GCLID at the first visit so it is there when the deal closes.
Services in this project
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Built and written up by John M. Williams, founder of It All Started With An Idea in Queen Creek, AZ. 15+ years and $350M+ in managed ad spend.