[ATU/M/ISK]

The most common sign I've found is if your significant other starts to suddenly become suspicious that you're cheating.

This has been the case with everyone who's ever cheated on me, and I've never cheated on my girlfriends in the past.
It obviously may not be the same for everybody, but they don't forget what they've done, so they start trying to look for or notice signs that you are.

This could also be a cop out to break up with you before you realize what they've done.