Continuity mistake: When Palmer is revealed to be a thing, he shoots up to the ceiling. Windows is ready to use the flamethrower on him. He has the flamethrower pointed up to the ceiling, yet when Palmer falls to the floor, in that split second, Windows changes into a completely different position with the flamethrower. (01:23:35)
Continuity mistake: When Childs takes an ax to the supply room door where MacReady has broken into, there is a shot of him hitting the door high up. Then there is the shot of MacReady almost frozen. When the shot cuts back to Childs, the ax marks in the door are not consistent with the shot from a few seconds before. (01:12:50)
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