During the movies The Thing 1982 and its prequel 2011 the alien entity would take over an organism by replicating its host at the cellular level becoming genetic duplicates.
Of the few ways found to expose these alien beings included searing blood samples to test if it reacts in a defensive manner and attempts to escape damage (uncharacteristic of normal blood).
It was also found that the process of alien absorption rejected foreign objects from the bodies of the parasites host, effectively making that one way to tell if a human with known enhancements has been "replaced".
We know the alien can replicate canine and humans, therefore presumably all mammals. Perhaps even birds and insects.
My question is this: If the alien parasite were to successfully make its way to warmer climate and begin to infect the planets animals (human included) what possible ways would the human race have to detect alien creatures vs natural uninfected animals.
I envision the surviving humans in strongholds surrounded with flame throwers killing any animal that comes near. Those wanting admission would no doubt enter a screening room where the blood test could be administered. Blood sample tests are impractical for wild animals making safe travel neigh impossible.
I imagine some non human animals may be able to avoid the aliens sensing something wrong / unnatural but short of killing everything how can humans safely discern the difference.
Unless there is a way to detect these creatures there would be no practical way to fight back short of a mass nuclear cleansing and torching everything NOT confirmed clean. Scorched earth sterilization.
I have only seen the movie adaptations, perhaps there are other canonical clues that may bring hope to this besieged planet. Or are we truly doomed to assimilation?