Tip 1: Have a few questions that you ask at all the colleges; T26 has a good list but it'll be hard to get to more than 4-5 in a tour (or even sitting down with a random student at lunch). Asking the same questions everywhere gives you a basis for comparison after it's over.
Tip 2: Try to make the questions ambiguous/open-ended: EX. What the party scene like? Don't imply anything, just let the answer come out.
Tip 3: Listen to how the question's answered. Does it sound like it's straight off the web site or do you feel you're getting a personal response.
Tip 4: Listen more - Talk less. A follow-up question that leads them toward something you want to know is better than telling them exactly what you want and then having the student try and adjust the answer accordingly.
Tip 5: Pay attention to how you're treated before, during and after the tour. This is your biggest contact point with a university; if they can't be pleasant before you even apply why will they be nice once you're in?
Tip 6: Have a checklist with some (5-7) important factors: e.g., Location, Safety, Academics, Dorms, Girl/Guy Cuteness, Attitude etc. Immediately after the tour rate those factors 1-5. Don't talk about it, just react. After you've finished the answers then talk about it. You'll be surprised how the discussion will flow and how you'll identify what's important to you.
Tip 7: Go with your gut instinct. It'll be right more often than not.