If they never apologize. Apologizing is normal, nobody is perfect, but if you realize your partner never apologizes you'll probably also realize it's because they rarely ever take responsibility when something goes wrong. If they show up late, it's because their roommate misplaced their keys, the bus was late, they had to stop for gas, traffic was crazy, etc. If your feelings are hurt it's because you misinterpreted what they said, you gave them the impression that you actually would've liked it, they actually meant to do it another way but something else caused it to change. If they do something wrong it's because you drove them to it, you actually don't understand the situation, you're jumping to conclusions, what they did wasn't wrong in the first place, you're trying to control them by saying that that's wrong.

Sometimes it's obvious, but other times you get sucked into what they're saying and you don't see what's happening.