In the movie The Book of Eli, some of the characters look at each others hands the first time they meet. I don't recall any explanation of this behavior in the movie. What were they doing and why?
They looked at each other's hands to see if they were shaking.
At one point in the story it's explained that a side effect of eating too much human meat is a shaking of the extremeties.
They were checking to see if the other persons hands were steady, or were they shaking.
It can be caused by a genetic trait, and is difficult to catch otherwise. You basically need to be injected with serum or an endocrine extract from someone that has the disease.
Otherwise you can pick it up from eating meat from an infected human i.e. Cannibalism. See Kuru Disease for real world evidence of this.
Basically, if you were to chow down once on one person, you'd be very unlucky to get CJD. However the more - ahem - specimens you sample the greater the chance of contracting CJD. Multiply this by the number of specimens sampled by the specimen you're eating and the probability of contracting CJD increases.
Therefore in a society where cannibalism is common place, the chances of a getting CJD - and therefore having shaking hands - could be quite high.
So, if you tend to be of a nervous disposition or suffer from an uncontrollable tick, pray you don't end up on a post apocalyptic world where cannibalism is frowned on :)
To make things simple in the movie after the big blast from the some food supply was very low, so some people on the verge of starvation started eating other people. When you eat a human being it is said the you get the shakes on your hands.
To be safe and not be ignorant to the fact that someone was a cannibal they checked their hands first to see if they had the shakes. When you watch the movie look out for the people with shakes, for example the old woman with tea and you will understand certain scenes more.
I like the medical answer a lot, and it make sense not to serve cannibals. But I thought to look at a biblical meaning since the movie is very spiritual. Did some research and found this in the book of Revelations.
Revelation 13:16-17 King James Version (KJV)
16 And he causeth all, both small and great, rich and poor, free and bond, to receive a mark in their right hand, or in their foreheads:
17 And that no man might buy or sell, save he that had the mark, or the name of the beast, or the number of his name.