The good ones take a real interest in you, in your opinions, your activities, your friends and family, etc. They are not so focused just on the physical but relate to you all over as a real person, not some means to get their sexual needs met. Try going out with them for a while, several weeks, with no sexual stuff, and see what happens - are they still there for you or did they realize they're not going to get the quick sexual thrill they were only seeking?
you tell the good guys by listening to their behavior.
for example, a guy who wants to get in your pants will _say_ with words that he wants a serious commitment, that he respects how you want to move slowly. but his _behavior_ will say that he's always pressuring you to get physical, to come back to his place, to prove that you care about him.
there are things you can do to test a guy's interest level. most female "tests" for guys actually backfire and make things worse because there's no real plan. here are two tests that actually filter guys based on the guy's _behavior_.
1. you go on scheduled DATES with him. you _insist_ on scheduled DATES with him. this means no "hanging out" and playing video game or watching DVDs. "hanging out" is often an excuse to get on the sofa next to you so he can make a move.
2. you plan one DATE and he plans the next. but the secret is that you can't tell each other what you're doing ahead of time, and you have to do a new activity, event or location on each DATE. this is a way to see what his hobbies are, and to see his behavior in different settings. this method proves that you're willing to put in some effort towards the relationship.
I think you should be with them for a few months before you even think of doing anything physically. If they wait that long then they're a good guy, if they don't then its obvious they only wanted one thing.
There is no RIGHT time for that kind of thing. I've been dating my guy for maybe a month now. He isn't officially my bf, but there is no one else I'm involved with. I had sex with him the second week of dating him, it sounds stupid and irresponsible, but it FELT right to do it at that moment. Do I wish we waited a little longer? Yes. But I would never take it back now, it was too amazing. I was worried that because he got into my pants so soon, he would maybe lose interest in our relationship and not want to hang out with me anymore. But it's going very smoothly and there's no doubt in my mind that he really wants me because he likes me. In fact, we completely refrained from talking about sex until after we actually had sex. Now we talk about how it was, what we liked, what we could do better. I love it because being able to talk about sex has made it even better. So don't automatically think he may be up to something, I was that way too. I didn't agree to go out with guys just because. But when he came around, I KNEW there was something about him. Follow your gut, your heart, all that stuff :) have fun, be safe. Hope this helped!