An ex use to say that the size of the thumb and thumbnail was a good indicator of cock size. Seems that there might be some correlation but my current BF has a large thumb and thumbnail and he's very average in cocksize. I on the other hand have what I think is an average thumb and thumbnail and my cock is much larger than my BF. A friend of mine has big hands, including his thumbs and thumbnail and a HUGE cock! I now am possessed and watch thumbs constantly. I get boners when I see large thumbs and imagine what might be hiding below.
There's also the theory, that looking at the back of a guys hand - that area from the wrist to the little finger - can give you an idea of a guys ass hair status. If the back of the hand is hairy, his ass is hairy, if the hand is smooth, his ass is smooth. So far, that's been almost 100% accurate. Check it and see.