Not all are impossible to tell at all, when it's genuine.
Those who might have seen the gorgeous Alisa, ex Dulwich Hill for example, may have seen her experience a massive orgasm. If you managed to coax her into a daty session, it may have taken her 10 or 15 minutes to get her there, but when she came she'd ignite with shuddering and have to push you away, almost at risk of falling off the table if you kept on tonguing her because she'd be then so sensitive then around her clit after cumming