well, depends on the player. Aggressive players it is very difficult to spot, while overly passive players it is usually blatantly obvious. Also, read the board. People will rarely check-raise top pair unless it is an ace or a king (less chance of being outdrawn by higher pair) but they will frequently check-raise with a set because there is less a chance of them being outdrawn. So with top pair, if someone calls two big bets and then raises you, once in a while it is best to lay down top pair, just remember that sets are relatively rare so don't lay down top pair too often or people will exploit that in your play.