Answer:
by using his theories and information from others. The second time he sees the hitchhiker he says that the fast truck could have picked up and dropped him off before Adams got to the destination. He asked questions at the gas station to try and understand the raindrops on the hitch hikers shoulders to determine if someones will be hitchhiking at Pennsylvania turnpike. He also tries convincing himself it's a coincidence. He later decides it's a phantom or ghost when he picks up the girl he tries discussing how the hitchhiker might be able to arrive to places faster than others in a car.
Explanation:
