Visible crew/equipment: After MacReady says, "The generator room," toward the end of the film, he, Garry and Nauls walk down a flight of stairs. Then Garry and Nauls walk down a set of three steps whilst Mac is holding a flare. As they walk the shadow of a guy holding a boom mic is clearly visible on the floor. (01:30:05)
Visible crew/equipment: At the end of the film, where MacReady is about to fight 'the thing' you can see the shadow of a guy holding a boom mic, on the ground just behind him.
Visible crew/equipment: When Nauls is in the kitchen cooking and the camera moves towards the room next door, the shadow of the camera is visible on the right wall. (00:14:35)
Visible crew/equipment: When the "Palmer Thing" attacks Windows it lifts him up in the air. As it does this you can easily tell the monster is played by a member of the stunt team by the dark green T-shirt it's wearing. Palmer's clothing is different before and after this shot. (01:23:46)
Visible crew/equipment: When we see the huge dog alien, right before Childs burns it, a crewemember controlling the puppet is seen on the center of the monster's head.
Visible crew/equipment: Bennings tells Nauls to turn down the Superstitious song and Nauls skates over to the radio, but then doesn't turn the radio down at all. The shot then cuts to a shot of the medicine room which then continues as a panning shot and pans into the next room. Just as the camera stops panning and the shot remains still, there are a few stage lights visible reflected in one of the cabinets on the right hand side of shot. Those certainly aren't lights because all of the lights in the room are switched off. (00:15:10)
Visible crew/equipment: When Windows is in the radio room, he is trying to communicate with McMurdo. Doctor Blair then enters the radio room and approaches him, tapping his shoulder. Windows then pulls his headphones down from his ears and has them placed around his neck and he then says to Doctor Blair, "Nobody." Just as he says this, there are two white screens visible in his glasses. (00:11:00)
Visible crew/equipment: When the "Thing" monster bursts out of Norris' chest it rises up to the ceiling and grabs onto an oblong-shaped ventilation shaft. MacCready steps forward with his back to the camera (getting ready to burn it) and in this shot if you look at the adjacent pipe on the ceiling you can see the hand of a crew member. It looks like the crew member is there to steady or grab the monster puppet if it falls. (01:15:45)
Visible crew/equipment: Just after Nauls is told by Bennings to turn down the music the film cuts to a tracking shot going towards the rec hall. At the very beginning of this tracking shot keep an eye on the lower half of the medical cabinet, and you can easily see the dolly that the camera is being pushed on. If you look just above the dolly you can make out the blue sweater sleeve of the crewmember pushing the camera as well. (00:14:55)
Answer: The death of Fuchs is probably the best answer to this question, as it appeared that Fuchs burned himself alive before the Thing could assimilate him. It's also possible that Norris suspected he was infected before he transformed. There is a scene in which we see Norris, who is alone, suddenly wince in pain, surprised, and grab at his chest, but he continues functioning normally thereafter. Following the altercation with Mac, Norris collapses and becomes unresponsive, until his chest cracks wide open and bites off Copper's arms. Also, in the blood test scene, Palmer's facial expressions appear to betray his secret, but he was already fully transformed at that point.
Charles Austin Miller