Committing a .env file is the leak that cannot be un-witnessed. The .gitignore Checker asks the one question a repository owner should be able to answer instantly: if I run git status, would my private env files show up?
It applies Git's ignore semantics to your .gitignore — pattern anchoring, trailing-slash directories, negation with !, and the last-match-wins ordering — and tests a candidate path you name, typically .env or config/app/.env. The verdict is one of IGNORED, TRACKED or EXCEPTED, with the matching rule shown.
The riskiest case, a negated !.env reopening an ignored file, is called out explicitly. All matching runs in your browser.