How can you tell if someone is cheating on you?

If they seem distant and preoccupied for no apparent reason, if they have unexplained changes in mood (for the better or for the worse), if they have to work lots of overtime or suddenly have lots of places they have to go all the time, or if they suddenly become much more concerned about their physical appearance, especially after they've been involved with you for a long time and have let themselves go a bit. I think that last one is a big clue that something is going on.