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.
What this checks, precisely
It looks for the viewport meta tag, the one line that tells a phone browser to lay the page out at the device's real width. Without it a phone assumes a desktop page and zooms out. This is a binary, structural check and it is honest about being one: it proves the site was built with phones in mind, not that every element behaves.
Why Google turned its own tool off
Google retired the Mobile Usability report, the Mobile-Friendly Test tool and its API on 4 December 2023, on the grounds that mobile-friendly is now simply how sites are built, and pointed people at Lighthouse and PageSpeed Insights. That is reasonable for developers and useless for a business owner who wants a yes or a no, which is the gap this fills.
Mobile-first indexing means the phone version is the version
Google indexes the phone version of a site, not the desktop one. If content is hidden or broken on mobile, that is the version being ranked. A site that looks fine on the owner's laptop and bad on a phone is not a cosmetic problem, it is the version Google is judging.
How to check it properly, in thirty seconds
Open the site on your own phone and try to do the thing a customer would do: find the phone number and tap it. If you have to zoom, if the number is not tappable, or if any part of the page scrolls sideways, the site is failing the only test that matters regardless of what any tool says.
Common questions
Is this the same as Google's Mobile-Friendly Test?
No, and it cannot be: Google retired that tool and its API in December 2023. This checks the structural signal, the viewport declaration, which is what that tool checked first before it ran a full render.
It says yes but my site still looks bad on a phone. Why?
Because the viewport tag is the floor. A page can declare it and still have a fixed width table, an oversized image or a menu that does not open. Open it on a phone and look. That is the check no tool replaces.
Does this affect my Google ranking?
Google indexes the mobile version of your site, so a page that renders badly on a phone is being judged in that state. It is one input among many rather than a switch, but it is one of the cheaper ones to fix.
Do I need a separate mobile site?
No. Separate m. sites are long obsolete. One responsive site that adapts to the screen is both the standard and the thing Google expects to find.