An excerpt from Tangyin bishi 棠陰比事, a 17th-century collection of great Chinese court decisions compiled by by GUI Wanrong 桂万栄 and translated for Iwanami by Komada Shinji 駒田信二. The name means "Comparisons of [judgments made in] the shade of the pear tree", the pear tree apparently being the place where legal complaints were legendarily/traditionally heard.

There was an old man who lived in Henan during the Han dynasty. He was over eighty years old and quite wealthy, but had no sons, and his only daughter had already married and left his household. His wife having died, he took another, and before long a son was born to them. Then the man himself died. His second wife began raising their son on her own, but after a few years had passed, his daughter reappeared, claiming that this new child was not truly her father's son, and demanding all of the money that the old man had left his widow.

Bing Ji 丙吉 heard the case. Recalling the common knowledge that the children of old men are highly sensitive to the cold, and do not cast a shadow even in the sun, he gathered together many children of the same age and dressed them all in identical tunics. The child at the heart of the case shivered while none of those around him did. Bing Ji then had all the children moved outside to sit in the sun, and observed that the old man's son was the only one who did not cast a shadow.

In this way it was decided that the old man's widow and her son should receive their entire inheritance.

I think Bing Ji may have been confusing "children of older men" with "vampires". Still, if you grant his premises, it's hard to fault his scientific method. And that scene where a bunch of identically-dressed toddlers are herded outside to cast shadows is priceless.

Related: Reinterpreting the Law in the Song, by Colin Hawes; Traditional Chinese Jurisprudence and the Supernatural: Can Ghosts give Evidence?, by Terry Kleeman.