Charles Austin Miller

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

Answer: The Norwegian malamute freely ran around the American base for hours before the kennel incineration scene. Doc Blair even asked the dog-keeper, Clark, how long the malamute had roamed around the base that afternoon, and Clark couldn't give an exact amount of time. It was easily long enough to infect more than a couple of hosts, as there was no single point of infection. Also, even before the kennel scene, the Americans went to investigate the Norwegian base and recovered a half-mutated humanoid corpse, returning with it to the American base. After performing an autopsy and blood analysis, Doc Blair realised that the corpse wasn't completely dead. The animated goo from the mutated corpse is what assimilated Bennings.

Charles Austin Miller

Answer: Burning isn't always a guarantee it is dead. They just think it is. The only permanent solution is blowing a Thing to smithereens as it doesn't seem to be able to put itself back together after.