i dunno how empathetic you are, but i can kinda do some brief eye contact with someone and sorta tell whether they're ok or not. i've never had to turn down a ride, but i've ridden with some creepy-ish ones, but they didn't do anything. then again, i'm a guy, and it's different i'm sure. just be ready to turn down a ride if they seem real sketchy, even if you've been waiting for days and are desperate. it's not worth being in a bad situation.
a friend of mine knew a girl who got in a truck and, after a while, realized that they weren't on the highway anymore, then the guy, in so many words, asked her what she'd do if he tried to rape her, and, grabbing her knife, she said "i'd fucking kill you because you're afriad to die and i'm not." he let her out and she had to walk, but she didn't get raped either.
just remember, when someone else is driving, you're in control of the situation. especially if you have a pocket knife and aren't afraid to use it. (hopefully you never will, i've never had to use mine for that purpose) a strong confidence is also good... people can sense that, and are alot less likely to try and take advantage of ya. -- and yea, it might not be a bad idea to travel with someone else if you're insecure or uncomfortable about the notion.
another thing to consider is the positioning of people in a car. the person behind the driver is in a key spot, he/she could fuck up the driver if they wanted to. similarly, the passenger-side back-seat rider could fuck up the passenger. on the same note, the passenger has advantage over the driver, as they aren't occupied with a steering wheel, etc., though they have to think about who's behind them. all things to consider, when you're riding with people, etc.. me and a friend, when driving (back in those having-a-car days), used to pick up some pretty gruffy wily dudes, and my buddy would get in the passenger-side backseat and let them sit shotgun, so in case they got a crazy idea, my boy would be behind him, as a precaution.
be careful out there-- use your head and intuition & you'll be okay.