Sometimes, it can be tough to tell.
But frequently, it's like shooting fish in a barrel. What are you women thinking?
One guy I met, he just never ever shut up. (Thank god for that girl it was a one-night stand.) Other times, the dude doesn't hold a job, has no money, doesn't pay any attention to her, calls 20 times a day, smokes a lot of pot, etc. Within a week or so, it's usually obvious.
Listen to your friends. Seriously. When they warn you, they are often on to something.