Setting aside the goods that Lizard Dude has brought to the table, this is an excellent question, and I dig the philosophical implications of its answer.
Ask somebody to write their name in the air using a finger. There are generally two types of responses: They either write oriented towards themselves, so that they can read it--or they write oriented towards the viewer. Some people claim that the latter response indicates a more considerate personality. I don't buy that, though, because I have known a few really inconsiderate people who readily pass petty tests like these.
On the other hand, if someone uses another person's finger to write, they are probably a psychopath.
