Balance it on a point. Of it falls on the different adjacent sides with even frequency it's fair and balanced. If it spins of its own accord, rises off the table, glows to the point that it hurts to look at and opens a gateway for unspeakable and unknowable evil to enter our world, you probably need to file one of the edges down.

The new demon living at my place instead of Noah aside... Turns out a few of my dice do actually have a bias. 2 of them towards 14, one towards 2, and another towards 4. all of the clear ones are perfect though.

hmmm now I want to do this...