Difficult question, even if you're not interested in her in 'that' way; always has so many implications... Soothest way to do it, regardless of your intentions is to make the assumption she has a boyfriend (as wikiwiki said); she'll correct you if she doesn't. It's an indirect compliment for her, too, and also implies you're interested in her without being TOO interested or direct.
Definetly shouldn't be done too early, as Soha said... on one hand you need to know, but in the early stage it shouldn't matter: get to know them and if "some big cunt" (quote:SashatheMan) comes up to you & is aggressive, he shouldn't be so insecure as to prevent his girl from socialising. ...Chances are if he were that protective/jealous you'd not have had a chance to talk to her becuase he'd not have left her side, anyway.