Arnauld If from F-Droid I can be sure
You can't. Many apps still use network capabilities and it isn't easy to know what they data they may be collecting. F-Droid is generally more trustworthy in the sense that most of the apps in the main repository are open source and have a developer's name tied to their app.
If you find the same app from the same developers, one on F-Droid and the other on Play Store, chances are those two apps aren't entirely different regarding tracking. But as always, do your due diligence.