Alphitomancy

Alphitomancy (from Greek ἄλφιτον "alphiton", 'barley', and μαντεία "manteia", 'divination') is a form of divination involving barley cakes or loaves of barley bread.
When someone in a group was suspected of a crime, the members of the group would be fed barley cakes or slices of barley bread. Supposedly, the guilty party would get indigestion, while all others would feel well.