How can you tell when a guy or girl is cheating?

I am going through a major breakup now because I just found out he was cheating. He stayed out late, magically forgot his cell phone at the office, dressed cuter going to work, stopped looking in my eyes, and lied all the time. Lies, lies, lies. They have an excuse for absolutely everything. If she is cheating, catch them. Just happen to show up where they are. Or hire Joey Grecko from Cheaters.

They don't call nearly as much as they did before, they stop saying I love you, less sex, always gone or coming home really late, have excuses for everything, don't do special things anymore, don't want to spend quality time anymore, and so forth but those are the main signs of a cheater.

I've had this happen to me. They generally act differently than normal, and become very secretive. Just acting suspiciously in different ways. And communication becomes much more difficult because of the lack of intimacy between you. I guess to sum it up, look for big changes in their personality.