Below are listed some warning signs that should make you wary but rather than focusing on single acts, look for patterns of behaviour that show control, restriction and disrespect. No-one should be frightened of their partner or prevented from making choices about their life.
Remember also that abusers are often very charming and convincing to everyone - including their partners, until the abuse starts - and then they often continue to be very charming to everyone else except her.
This often has the effect of making the woman think 'oh it must be me, it must be my fault', especially since the abuser is usually telling her it is. It can also make her feel awkward about telling other people because she won't seem plausible because they only know his 'nice' side.

The single biggest warning sign is:

1)If he's been in a violent relationship before. Abusive men rarely change.

Don't make the mistake of thinking 'it will be different with me - she didn't treat him right'. It's also worth remembering that although there are some men who are abused, almost without exception, every abuser claims that he was really the victim.

Other possible warning signs are:

1)He puts your friends down and / or makes it difficult for you to see them.
2)He loses his temper over trivial things.
3)He has very rigid ideas about the roles of men and women and can't / won't discuss it reasonably.
4)His mood swings are so erratic that you find yourself constantly trying to assess his mood and only think in terms of his needs. A healthy relationship has give and take.
5)It's difficult for you to get emotional or physical space away from him - even if you directly ask for it. And if you do get it, he 'grills' you about where you've been and who you were with.
6)He criticises you all the time - about your weight, your hair, your clothes, etc.
7)He makes all the decisions in your relationship and ignores your needs or dismisses them as unimportant.