Rhythmic contractions of the muscles in the pelvic floor -- specifically the anus.
If you spend time going down on a girl and you gently slip your finger in her pussy, you should feel the walls stiffen slightly, and eventually throb four or five times as (if) she cums. This can be faked, so I prefer gently sliding a slippery finger just over the bum hole at the key moment. (Careful though, if she pulls away she doesn't like being touched there). I don't think anyone could fake the rhythm of a true cum.
Many ladies can cum more than once, so ease off for a minute or two and she'll soon be off again. I find this a huge turn on.
Having said all that, many WGs won't or can't cum with a client, so just enjoy yourself.