This app is 100% open source! You can find the source code here:
https://github.com/1nikolas/play-integrity-checker-app

This app shows info about your device integrity as reported by Google Play Services. If any of this fails could mean your device is rooted or tampered in a way (for example having an unlocked bootloader).

Note: Google has a 10k requests/day limit. If it stops working it's probably that.