Do you mean if they like someone else? If so, I would say, let's say for instance, if they had a crush on someone at work or school. They would probably every so often bring the person up hear or there. Maybe mention a joke they said or a funny encounter. It wouldn't be all the time, just every so often. If they like someone then that person is most likely on their mind a lot, so it is natural for them to bring up the person occasionally just to release the urge of wanting to talk about them. Trust me, us girls love to talk about the guys we like so to have to keep it all in is extremely difficult.
Ohh dear! I seriously hope this isn't happening to you dude.
I would say, checking their mobile phones, smiling at messages, being secretive when you ask who is was etc etc. Also bringing him up in conversation, "Oh! Mike did this, or said this...it was so funny - he's a really great guy!" sort of thing, also making an effort to be around them, or to be seen by them, being in places they know that guy is going to be etc etc.
If you have doubts that you girlfriend likes some one else, then sit down and talk to her about it, ask her if she is bored of you, how you can spice up things. But importantly ask her if she still wants you and wants to be with you, if she isn't definate then there is no hope. Good luck friend!