Hearing the sexual failings of their past lovers will only galvanise your status as the one who's now in pole-position, and give you an insight into how to please them better. On the other hand, if they're wistful for the steamy sessions of the past, you know you're dealing with someone on the rebound. And if they talk about their previous partners in a way that's designed to make you feel inadequate, then you know you've picked a nasty one and ought to tell them to get lost.