How can I tell if a guy is up to no good?

Congratulations for not giving in; as bitter as you are, at least you didn't give in your virginity to these guys because I know quite a few girls that have fallen victims to the courtship of players, and after the players got the sex, they left, and needless to say, these girls were devastated. So you're doing very damn well, as bitter as you might think you are.
Unfortunately, you always have to be on guard because I know players that have courted a girl for 8-10 months,and once the girl gave in to their line of 'if you love me you'll have sex with me', they left the girl in no time. I was amazed myself, so there is no perfect guide to tell what makes a player.

With that said, the easiest sign of course is if the guy is pushing for sex right away; however, if you're not giving it up, he could easily have a friend on the side. That's why if you run across a guy that has a few close female friends, and they tend to be the social butterfly, then you have just increased your chances that the guy is up to no good; granted, there are a few guys that have a few close female friends, but if this is the case, I will always say to be careful with these guys; if they have so many female friends, then is easier for them to go and 'meet' women; or girls are attracted to his personality, and thus, the chances of him getting together with one are higher than lets say with a guy that doesn't have as many female friends.
Anyway, to pointers that maybe could help you:
-they are vague, and this is a big one. When you ask him about his relationship experience, or what are his thoughts in getting committed, he will reply with 'oh, id love to get into a relationship but right now I'm concentrating on work, school, military' or anything else. From the sounds of it, you will say, well, that sounds very reasonable; he is trying to improve himself; however, the more you ask questions, the more generic answer's he will have. These vague lines always worked for me and girls never knew what my real intentions were cause they though my concentration was in getting ahead in my career. :D
-He calls to 'see what's up, see what you're doing, see if you maybe want to 'cuddle' some'; when you say that you're busy or whatever, he immediately dissappears.
-that leads to, calling only when we need 'something.
-another dead give away: always has an excuse for not wanting to take you out; maybe he'll go out once a week, and when you suggest you want to do something, either he is too busy, or he says, "that s a good idea.why don't we do it on such and such date," and on that date, you're actually busy with stuff you can't get away from. Now we have the excuse to say, well, see? I asked you out but you're busy! :)
-of course, telling you tons of emply compliments
-check out other women when ure with them
There is always the advice not to give it easily, but like I said, I know of players that waited months to get it and they left as soon as they got it. WEll, I ran out of space :)

If they are into drugs, have a bad attitude, flirt with other women, are a player, don't seem interested in you or your life, stare at other women when they are with you, make sexual advances, move very quickly/are touchy feely too much or too fast, then you know they are no good.