How do you know if your guy likes someone else?

Well if he pays less attention to you that's a start. If he's the kinda guy whose always on you and then all of a sudden he doesn't care what you do that's also a sign. Yet if he becomes extremely paranoid about what your doing, more than normal, than that's also a sign he likes someone else. If he doesn't answer your calls or texts when you think he's up to something... he possibly is.