On the more subtle side, he won't make it a point to hang out with you or talk to/text you. He also won't be as enthusiastic when he does talk to you. As a note, you CANNOT gauge enthusiasm in text messages.
On the more obvious side, you might hear about him complaining about something you do or he'll get upset easier while talking to you on a consistent basis. He might even start ignoring you all together.
Go to the guy and tell him to tell his girlfriend that he has a crush on you and that he thinks they should get over each other.
Move on find another guy
if they loose interest in you
forget him or make him admit it
if he doesnt flirt with you 24/7 then he doesnt like you...(=
If he doesnt talk to you, takes no notice of you, then he doesnt like you. :(
tell him you need a vacation or should go out more often
If you are apart and he behaves "like he doesnt love you anymore" then it is over and the relationship isn't "salvagable"
tell him you dont like him anymore
tell on him or not like him anymore
ask him
ask him if he would have a threesome with you and another guy and if he goes for it then he doesnt really love you anymore