From my experience BBFS happen when you and WL/ML are click and very horny. Just like you they are also human so sometimes they can get carried away. If
- She's soaking wet
- She keeps doing pussy/cock slide
- She keeps wanting to kiss you
- She play with herself
that usually means you can BBFS if you wanted to. However, its still up to punter whether you want to go ahead to do BBFS which will come with all the risks.

I don't really believe that if a girl do BBFS with you means she do BBFS to lots of people or if the girl don't want to do BBFS with you means she never do BBFS with anyone. There must be some guys who the WL/ML attracted to that she just couldn't resist or she might be very horny.

Personally, I have done it myself 4x in my entire punting life, it happened when I got carried away(the girls were very HOT). The regret always come after and the worried for the next 3 months was very frustrating. So far I'm lucky not to catch anything.

Just make sure that if you do BBFS, it was a mutual agreement. Don't stealth and don't force the girl to bareback if she doesn't want to, that's rape.