I leave an envelope lying in plain sight at the beginning of the session. I do this for anyone I meet with, whether they ask for it or not.
I recall three times, when doing this, where the escort opened the envelope before the session, counted the money, and pocketed it beforehand. One of them carelessly dropped the envelope on the floor afterwards, which should have been a tip off.
All three of those sessions ended early because the escort refused to do things they agreed to earlier.
I have had escorts insist on prepayment, at which point I tell them my aforementioned policy. None of them argued. Surprisingly, only one of those was a bad experience, and it was the one where the person grabbed the money and dropped the envelope, as I mentioned before. And it was by far the worst session I ever had.
I ought to write a monograph on this someday...but there seems little point in kicking someone out immediately after they have your money.