How do you know when someone is lying?

How do you know when someone is lying?

How the ancient Chinese did it.

The Chinese used rice. An examination for truthfulness might go something like this: 'Is your name Chiang?' (They know the guy's name is, in fact, Chiang.)
'Yes.'
The interrogators hand  Mr Chiang some rice. They have already counted the number of rice grains.
'Ok. Put this handful of rice in your mouth. Hold it for three seconds. Spit it out.'
Then they count how many rice grains come out.
'Did you steal the chicken?'
'No.'
'Ok. Put this handful of rice in your mouth. Hold it for three seconds. Spit it out.'
Again, they knew how many grains went in, and they count how many come out. If more grains come out after the question about the stolen chicken than came out after the 'easy' question, where the suspect truthfully gave his name, they know he is lying. How? The stress of being caught lying makes the suspect's mouth drier. Fewer grains stick. More come out. Mr Chiang stole the chicken.