In the West, Maverik stations have the best road food. They actually cook most of it onsite rather than have it delivered. (Their jalapeno cheese bread is awesome.)
I do love little mom n' pop places, though. You can usually tell at a glance whether it's going to be clean and healthy the moment you step in. If it's near a normal meal time, look for service trucks in the parking lot. A cop car, a phone truck, a construction pickup or two. If the locals regularly eat there, it's probably going to be very good.
