So you mean that do you have to RP and find out their name or just say the one above their head.
Personally (I don't know if there is actually a rule on this) I don't mind civis calling me by my name when just patrolling around the cities e.g. Hey HDGamer, How you doing? However in a RP environment such as a crime going on I would avoid ID'ing officers in this way as it is might fall a little close to metagaming.