Ohhhh, tricky.
As a general rule, a sign you're going to be broken up with is when he starts becoming more and more distant. If he's not answering calls, not talking to you, and not seeing you as much, something's up.
Also, from the questions and responses I get, cheating also seems to be a pretty common one, but I'm going to give your guy the benefit of the doubt.
-It's not the same if he is there but appears to be just going through the motions. Like there is no reason for him to be there anymore.
-Everything is an impossibility because it takes too much effort on his part.
-You are no longer key to all his plans.
Ouch. Ditto on all of that..... And God does it hurt.
When he says "I'm confused," that's a sure sign he's on his way out the door.
When he says, "I'm confused," it's a sure sign that a) he's cheating and b) he's on his way out the door.
Is this usually true regardless of how long the relationship lasted?