This is a great question. The dice come in 3/4 inch and 1 inch cubes. These are a usually only a few mm off being square if any due to the fact they are hand finished and crafted.
With that being said. I have ran a simple test myself and did 100 test rolls. The results were pretty even across the board. Even though I am not a scientist I conclude they are just as random as any store bought dice could be.
Thanks for the question.