I'd suggest trying the salt water test. You just fill a cup full of water and add a ton of salt (I mean a TON) until the die will float. Then just spin the die a bunch of times and see if it floats with the same number up repeatedly.
I keep meaning to try that but am lazy. (And I have a lot of dice I could test.)
I tested most of my dice when I learned about this and can't count how many I threw out. Dice with swirly colors are the worst.
I think airjudden wrote somewhere that he plays D6 games with a big bowl of many various D6 dice; each time he needs to roll, he grabs an arbitrary die from the bowl, rolls it, then puts it into a second discard bowl.
I.e. rolling a new die each time for each roll. A clever way to balance out any biases (if we assume that a bunch of dice from different sources will not all have the same bias, but have various different biases) (it seems a reasonable assumption).