Russell is the human.
Russell noticeably runs out of vodka earlier in the movie, but then has a full bottle of a clear liquid at the end of the movie right after he uses fuel to create the explosion.
When childs takes a sip of the vodka that is actually gasoline and doesn't react, that was Kurt Russell's test to see if childs was the thing or a human.
Childs is The Thing.
This is such a good theory. I don't know where I heard it, but whoever said this, I've been thinking about it for years and it has made me appreciate this movie even more than I already did. I recently saw it in the movie theater for its anniversary and it was so cool.