They have a blue marker above their head to alert the player to this fact. There are six girls found around the map that are dateable. However CJ's girlfriends, just like in real life, require upkeep and pampering, and the right care for their personality and tastes. The main reason to date a girl is to get a special bonus in exchange.
San Andreas introduces the ability for the player to pick up dates and go out with them.
