I've had this occasionally as well several years ago when I would occasionally get upgraded on United and looked like I was 15 at the time. Usually it would just be a simple and polite 'may I see your boarding pass ... thank you', which I've never had a problem with.

However, if a staff member would look towards me in a line and say 'this line is for first class passengers only' (or whatever), I would over-enthusiastically say 'sounds great!' and nothing else until they actually ask to see a boarding pass or ask me to move. If they are going to be passive-aggressive I was happy to be as well, although I would never say anything impolite or ignore them.