No signup, nothing stored. Checking somebody else's site is fine: this only reads what every visitor's browser already downloads.
This checks the page your address serves to a normal visitor, the same way a browser would, and it opens any Google Tag Manager container it finds and reads inside it. Nothing is stored, no signup is asked for, and the answer is the same one you would get by opening the page source yourself with more patience than most people have.
How to find a container ID by hand
View the page source and search for GTM-. The container ID is that prefix plus a short code, and it will appear twice, once in the script in the head and once in the noscript iframe just after the body opens. If only one of the two is present the installation is incomplete, which is common and mostly harmless, though it does mean visitors with JavaScript disabled go unrecorded.
Why a container makes tracking harder to audit, not easier
Everything good about Tag Manager comes from the same fact: the container is loaded at runtime and its contents are not in your page. So the pixel that your checker says is missing may be live, and the old tag you believed was removed two years ago may still be firing on every page. The only honest way to audit a site with a container is to read the container, which is what this does.
The tag nobody remembers installing
Containers accumulate. An agency adds a pixel, leaves, and the tag stays. A one week campaign heatmap script is still loading three years later. Each one costs page speed on every visit and, for anything that sets cookies, each one is a consent obligation you are carrying without knowing. Opening the container and reading what is in it is usually the fastest speed and compliance win available on an older site.
More than one container is a real problem
Two containers on one page means two sets of tags, potentially the same tag twice, and conversion counts that are inflated in a way that is very hard to spot. It happens when a theme ships with its own container and someone adds another. If the check finds more than one ID, that is worth resolving before trusting any number in any dashboard.
Common questions
Can I see what tags are inside somebody else's container?
Partly. The container file is public, because the browser has to download it, so what loads can be observed. That is what this tool reads. The friendly names, the triggers and the version history live in the Tag Manager account and are private.
Is Google Tag Manager the same as Google Analytics?
No. Tag Manager is a container that loads other tags. Analytics is one of the tags it commonly loads. A site can have either without the other, and this page reports them separately for that reason.
Does Tag Manager slow a website down?
The container itself is small. What slows a site down is what has been put in it, because every tag is another script the browser must fetch and run. A container that has been collecting tags for years is a common and largely invisible cause of a slow site.
Do I need Tag Manager at all?
If tracking is set up once and never touched, no. If you run ads, test things, or want to add a tag without waiting for a developer, it pays for itself quickly. The cost is that it must be maintained, and an unmaintained container quietly becomes a liability.