Even seeming really sad that the old relationship "hasn't worked out how I'd hoped" if coupled with other warning signs, can become a sign.
being aggressive generally, even to men down the pub or road rage could be an indication.
Checking up on you, mine used to walk me to and meet me from work which at first I though was so lovely, but actually he was just keeping tabs on me.
Subtle changes to what you wear or how you look. In the beginning mine would make little comments about what I should wear. If we went anywhere with his friends or family he'd choose my outfit because "so and so is a bit of a perv" or he would make sly put downs about how this one outfit made me look fat, or this one pair of shoes made me feet look funny. Or he'd make it clear what his preference was and I'd dress that way to make him happy.
Also seeing how they are when they're drunk, drink doesn't make a person abusive but it lowers the inhibitions that would usually keep that abusive side from coming out in public.
And irrational jealousy. That's a fairly easy one, if they tense or display displeasure at you talking to someone of the opposite sex. And for any reason. There's nothing wrong with having male colleagues, friends or acquaintances.
stuff like that can all be indicators of a potential abuser x