A person's hand is checked for the shakes to see if they're a cannibal. I've seen it before in the post apocalyptic genre of films, so of course, I had to see where it came from.
While not a full proof test, this little tidbit does have a factual basis.
Kuru disease, discovered in the 1950's, is a fatal brain disease that can come from cannibalism. (Only transmitted through cannibalism, though there is no guarantee of contracting it.)
It's a relative of Mad Cow disease, which is believed to have come about from feeding cows, well, cows.
It kinda works the same way - the little bugger eats your brain, creating loss of muscle control, insanity, and eventually death.
One of the onset symptoms is the tremors in extremeties, namely the hands and feet.
I like it when a trope has basis in fact, and this one is true.