try it through normal conversation... like: ,,What were you doing in summer?" ,,I was in Italy on holiday." ,,Yeah, what was it like?" ,,I was just with my boyfriend, so awesome." ... some of them will say it themeselves... or you can ask - ,,Were you with your family?" ... then she can tell you that ,,No, with boyfriend." or plainly ,,Yes." but you can ask in other topic... also, I don't see a problem using word boyfriend in your conversation, only asking directly feels rude...

other way is through friend that knows the girl you want... he/she can mention it in conversation about the girl or you can ask indirectly...