
Visible crew/equipment: At the Quidditch match, after Hermione sets Snape's cloak on fire and someone shouts, "Fire! You're on fire!" there is a close-up of Harry with his hands gripping the Nimbus 2000. In this shot, the wire rods which are attached/taped to the broomstick, near Harry's fingers, that lead under both of his red sleeves, are entirely visible. (01:21:35)
Answer: In the 7th book when Harry is looking into Snape's memory in the pensieve, Dumbledore mentions, "Keep an eye on Quirrell." This shows Dumbledore was suspicious of Quirrell and how Snape knew Quirrell may be up to something.