Hi Tom, thanks for taking an interest in this die and my dice-making.
I try as much as possible to make my dice fair in their design, in the case of the ossuary dice this meant two main design considerations: firstly the dice had to be symmetrical and the points of contact with a flat surface even, I achieved this by using hemi-spherical skull tops as the points of contact and rotating the skull design to spread their slightly off-centre weight distribution evenly; secondly the dice have to be evenly weighted and this is a trickier problem that I try to mitigate by varying the depth of the indices on the faces of the die.
Obviously this is rarely a perfect process, and I can't guarantee fairness for a die with such a dramatic design. But I can honestly say that I've done everything I consider possible to try to make them fair.
As to testing - aside from a dozen or so rolls after receiving a test-print, that would only show up a gross problem in fairness - I don't _test_ my dice for fairness.
If a die is grossly unfair, please let me know as much about the problem as possible (the circumstances, how reproducible the unfairness is etc.), and if a fix is possible I'll do my best to correct the design.
I'm not in the business of producing casino dice (which are rigorously tested for fairness) I produce unusual dice that are supposed to be enjoyable to use in a social environment and I believe my dice are fair enough for use in casual gaming with little (if any) chance of them being described as 'biased'.
I hope that helps. Again, thanks for taking an interest!