When she’s working the thighs with a hint of TT then you know there’s going to be extras on the flip.
Wait til she makes an offer and then negotiate from there. Let her know what you’re after. But if you’re after bbbj, you’d want to make sure you’re clean (take a shower before the session) to maximise your chances. Also, if your bald/old/ugly/stink then just go to an FS shop. You’re at least guaranteed CBJ and FS there even if you are a 1/10.