Your question raises a few possible answers. I'm guessing you're saying you have a crush on someone but don't know if he's a womanizer. So you're wondering how to tell if any guy is a womanizer.
If that's the case, I suggest you could be asking others who know him. Has he had lots of girlfriends? Do his relationships last very long? You could also be asking him the same questions.
But maybe your question is slightly different. Perhaps you already know the guy is a womanizer but you still have a crush on him, and you're asking me how will you know if he likes you. In that case, I'd be suggesting it doesn't matter if he likes you or not. The guy is obviously trouble and destined to break your heart. So I don't think you should waste time and effort on him. Find someone else to have a crush on.