Engage passengers in conversation while observing their eyes and body language. It doesn't much matter what is actually being discussed.
Questioning can be productive, too. One person observes the passenger as he is questioned, while the other asks the questions. But the questions need not be random or unimportant.
"Deep" questioning can be used to discover who is telling the truth and who isn't. Ask the passenger where he is going. You ask the passenger a series of questions, each of which is based in part on the answers to the preceding questions. For example, if he says he's going to see his aunt, ask what his aunt does for a living. When he tells you her job, ask which company she works for. And so on. For each answer you get, ask a new question that requires more details about his trip. A person telling the truth will have no trouble answering any of the questions, and his answers will be consistent if the questions are repeated. But a person who is lying will eventually run out of memorized, scripted answers, and will have to make things up, at which point inconsistencies can easily be spotted. Best of all, the method need not be kept a secret, because there's no way to get past it. It works with just one passenger traveling alone, but it works even better with two people traveling together (provided that they are questioned separately).