I like truecar.com. It tells you what other people are paying in your area for the same car. It at least gives you a ballpark to shoot for. USAA uses it with its price guarantee car buying service (or at least it used to-we dumped USAA for cars a few years ago for being shady with us).
You probably won’t get the best deal using truecar, but at least you won’t get fleeced.