Here is a list, compiled from my long, ignoble dating career, that will help the young ladies or gentlemen spot a dud early on. (Below I use the pronoun "he" for brevity's sake, but I suspect many of these observations apply to jerks of the female persuasion too.)
1. He doesn't laugh at your jokes. This is a lame power play, and a subtle way of embarrassing you.
2. He is rude to your friends or family. If he's a nice guy and he really likes you, he would want to please the people in your life, not drive a wedge between you and them.
3. He stands you up, and not because his car broke down. He felt like doing something else instead. No brainer, right? Dump him.
4. He blatantly ogles other girls while he's with you. This is impolite and uncool. You're the one he's with. He should be paying attention only to you.
5. He lets his buddies make jokes at your expense, or speak rudely to you. Any guy worth hanging out with has nice friends, and won't let anyone treat you with disrespect.
6. He talks meanly about girls he's dated in the past. If he pulls out the "B" word about another girl when he's on a date with you, he'll probably say something nasty about you later on too.
7. He doesn't make eye contact with you while you're talking. There is shyness, and then there is rude disregard. Learn to tell the difference.
8. You find yourself making excuses for him, or you spend a lot of time trying to understand his behavior. A jerk acts like a jerk because he is a jerk. Nice guys don't need excuses made for them.
9. He's mean to animals or younger siblings. This is a sure sign of a bully. A good guy doesn't use his strength to hurt someone who can't defend themselves. Get this guy out of your life ASAP.
10. He bosses you around. "Don't do this." "Do that." A good guy asks nicely, says please, and doesn't expect obedience.
The sexier a guy is physically, the more likely you are to stick around past his expiration date. The best way a young woman can protect herself and stop wasting time on jerks is to think clearly and logically, look at the behavior, and not let her mind get too clouded by hormones. Easier said than done. But I do wish I'd had a list like this when I was a teen and even into my twenties, and I wish I'd run from the guys I describe above. Time is precious. Don't waste it on someone who doesn't deserve it.