Over at Make, Gareth Branwyn wrote about how to check the balance of your d20 dice:

Gamer Daniel Fisher used an old golf ball balancing trick to test the integrity of his D20 gaming dice. To set up the test, he mixed 6+ tablespoons of salt with 1/3 cup room temperature water in a small glass jar. By floating and spinning a die in the jar, he was able to see if it consistently rolled high, low, or was balanced.

Among other things, Fisher discovered that translucent dice tend to be more balanced, perhaps because you can easily see imperfections inside them (and wouldn't buy or use them). Finding out that a number of his D20s regularly rolled low or high in the water, he cut into one to see what might be causing the imbalance. Inside, he found obvious manufacturing imperfections, chalky areas where the die may not have cured properly. Later in the video, he puts the cut die under a microscope to get a closer look inside.