Your question is the main reason for this site: to raise your chances. Other than that you frequently can tell by the therapist or the environment (I saw a therapist once who had "NO HE's!" on a whiteboard in the massage room). If they're using a sheet for draping I wouldn't bother asking. Quietly asking once something like "Do you release?" if it appears it's not going to happen will always settle the question.

Will you get into trouble? I've been told "No" plenty of times and never been kicked out or anything like that. I think if you're nice about it (which many therapists tell me a lot of guys aren't) and take one "no" as the final answer you'll be fine.