There is an extensive answer on our sister site, Science Fiction and Fantasy, addressing this precise question.
Rather than copy and paste it as a whole (and it's very long) I will quote the basic answer and link the remainder below.
This is the million dollar question, as far as The Thing is concerned. Unfortunately, not only has the answer never been revealed, but the producer and director themselves say they have no idea what the answer is.
There are a number of fan theories - both men are Things; neither man is a Thing; only Childs is a Thing; only Mac is a Thing - but the "evidence" employed to support arguments in favor of one or both men being Things is unconvincing, and it is easy to poke holes in every possibility except "neither of them is a Thing".
Even worse for those who think they have discovered subtle (or not-so-subtle) clues that point to one or both men being Things, the director and producer have both said that they didn't include any such clues, and even they don't know who, if anyone, is a Thing at the end of the film.
Source answer by Wad Cheber