If they're not in a bike lane and following the rules of the road the bike should move around to your left when they see a right turn signal. If there's enough room between you (if you didn't just pass them b5 seconds ago) that makes it clear that both people see what the other is doing. If they're right on your ass so there isn't room to go around without a serious swerve then you have to wait anyway because there's not enough time to be sure they see what you're doing.