Factual error: As the gun-toting Norseman approaches the buildings, Garry smashes the single-pane window with his handgun. It is inconceivable that the glazing in a structure near the South Pole would be single-pane glass, that would provide minimal insulation and which could be broken so easily.
Factual error: When the infected Palmer starts his transformation into the Thing, he has a series of fits and the creature 'falls', or leaps upward onto the ceiling. Watch closely...you can see a piece of paneling also 'fall' upward, when it should have fallen down. (01:23:20)
Factual error: In the kennel scene, MacReady fires at least 7 shots from a pump shotgun. That particular shotgun can only hold 5.
Factual error: If you are at or near a pole, aka Antarctica, you either have 24-hour daylight, 24-hour night, or a kind of continual dusk-like condition. You don't have bright day and dark night at the same time.
Factual error: At the beginning, MacReady dumps a tumbler of ice and scotch into his computer, which spews a shower of sparks as it shorts out. No electronic device, not even an old 1980s personal computer, sparks like that when shorted out. It simply goes dead.
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