how to tell wich 3rd party apps are safe

If you are asking to see a list of third-party apps, that have been proven to where the app developer can access your personal data, nothing like that exists as far as I know. If you have a specific app in mind, I would suggest that you do an Internet search on it to see if anyone has reported anything related to it.

I would second stedman1's comments. However, it's really your call. If you didn't find any negative news about this app from an Internet search, it may be "safe." Regardless, caveat emptor.

Try https://roaringapps.com/

No, anything that bypasses the App store is not "trustworthy".