How could you ever prove that nobody is doing that? You can believe anything that way

One can't prove god doesn't exist either, but as someone who made some privacy-friendly front-ends, I tend to expect honest intentions. If you find one that suddenly asks for your login data or sets tracking cookie, sure, be wary, just as with any other site that asks for data they don't need (see: literally every cookie wall, because if they had good intentions, it would fall under one of the five other reasons to use personal data and they wouldn't need to fall back to asking for consent)