I think @Watchkeys has it nailed. There are so many potential behavioural red flags that could also be totally normal and fine behaviours dependent on context, so I think the most powerful red flags are your own 'funny feelings'.
Eg
Feeling annoyed / embarrassed / hurt by something they did but like you can't bring it up because you have a gut feeling it will 'rock the boat' or because you know from experience they react poorly to being told they've upset you or done something inappropriate.
Feeling suddenly surprised / confused / taken aback by something they say in conversation or during a disagreement, because you can't understand how their thinking got them to what they just said. Particularly if asking them why they think that does not clear up your confusion, possibly because their explanation is just what they already said to you, using different words, and doesn't add any extra information.
During or at the end of a discussion, feeling as though they're 'not really getting it'. Sometimes this can be hard to put your finger on. Maybe they've said they understand and agree or apologise, but there's something about their tone that tells you they're just saying that to end the discussion. Maybe they appear to be listening but they're disengaged - they don't ask any follow-up questions. You tell them things and they never seem to 'stick' or 'sink in'.
Feeling as though they're ignoring you, angry with you or looking down on you even if you can't quite say why.