It can be very difficult to tell. Usually, at the end of your booking, if she puts on her lingerie, stocking etc ready for the next guy, you know she probably did the same for you. If she stays nude, or puts on a towel or robe, then she may have a shower when you leave. Some may join you in the shower, others shower straight after you, while you dry off and get dressed. Of course, by the time you know what she does, it's too late for you!
Edit to add- still no guarantee that how she preps after you is how she prepped after the previous guy.