I've had many guys on BBRT contact me and when I check their profile it says they are Neg. Since my preference is to play with other Poz Tops, I politely decline the Neg guys. Many times, the Neg guy then replies that he is actually Poz and gives me an excuse, like he didn't change his profile status after he became Poz. Lately, I've had Neg Tops tell me they are on PrEP so they are comfortable BBing a poz bottom.
While you shouldn't trust a complete stranger 100%, I've also met a lot of great guys through BBRT and other sites. Always ask questions if you're not sure about something. If I ask a guy questions and I feel he is being evasive, I move on. There are plenty of guys out there to have fun with. And if I agree to meet someone, it doesn't mean I have to be his bottom if he or I don't feel comfortable or have chemistry.