In 1911, a young Hungarian student named Gyorgy Hevesy was living in England, in a Manchester boardinghouse. He got a ringing suspicion that the landlady kept serving the same meat over and over again.
Did Hevesy complain? He went a step further: he put a radioactive tracer in the food.
Did it work? Did it ever! After a few days, he took a sample of the meat, and the hypothesis was proven: it was radioactive! (Can we safely assume that the landlady didn't get her meat from a uranium mine?)
So the landlady fessed up, and Hevesy soon moved out. He went on to discover the element hafnium and win a Nobel in Chemistry. All great achievements, of course.
But proving your cheap landlady is a liar? That has to rank up there.