I had a friend in middle school who could, given ten minutes with any six-sided die, no matter how biased, roll it to any specific number with near-perfect accuracy. I was never able to find a die he couldn't do this with. Later, he died in a car accident.
Mathematics.
You toss a dice, and it lands "6." You repeat the toss. It lands "6" again. How many repeats of 6 are required for you to declare that the dice is biased.