Deary me, how naive are some people? There's absolutely nothing wrong with dominant/submissive sexual relationships, plenty of people enjoy dominating or being dominated and inflicting/receiving mild pain. It's only a problem if either of you don't consent or aren't enjoying it, if the pain is extreme (I would say mild pain involves spanking, scratching, hair-pulling, choking, biting), or if issues outside the bedroom seem to be crossing over. For example, if he's angry with you about something and you can feel that translates to the way he treats you in the bedroom, you feel that he's hurting you in bed not for both of your sexual pleasure, but because he's genuinely angry and actually wants to punish you in a more general sense.
Other signs, like everyone says, jealousy - obviously some jealousy is healthy, but not to the extent that he would want you to stop seeing male friends. Treating you as childlike - making you feel foolish, or praising you for doing as he tells you. Raising his voice (or his hand) to you, especially in front of others, as it shows not only violent tendencies but instability and unpredictability. And you need to go on your instincts - if the feeling you get when he texts you or you're going to see him is more worried nervous than butterflies nervous, trust what your gut's telling you.