Dice can be unfair in a wide number of ways. Here are a few.
They can be weighted so that one face lands down more than others. One approach to detecting such weighted dice is to burn them looking for metal residue in the ashes. Cheaters got around that by simply hollowing out parts of the dice to unbalance them. Both metal weighted and hollowed out dice can be detected by dropping them in a glass of water and watching to see if they tend to turn while floating or falling through the water. Cheaters got around that common test by using mercury weighting with a chamber in the center where the mercury resides when the dice is to act fair and a connected chamber near the edge where the mercury is knocked to by the cheater rapping the dice on the table before a roll. Now, casino dice are clear to make it easier to detect weighted dice, but of course cheaters still try to put bits of heavy metal under the painted dots!
Dice can also me made in a fashion so that the tend to roll off some sides more easily. This, naturally, is more of a problem with dice that have beveled corners. Casino dice have nice sharp edges.
Instead of being rounded, some sides of the dice can be cupped, causing a suction cup effect that causes the dice to roll less easily off of one side.
Dice can be manipulated by narrowing them in one or more dimensions. Non-cubic dice may have just slightly smaller faces on some numbers or slightly slanted faces, making them roll in a non-fair fashion.
In the army, dice games are often played on the surface of a bed's blanket. Cheater put a small little burr below the surface in one of the dots to make it catch on the blanket fabric.
Finally, there are "joke store" dice with bad numbering, for example no single dot face.
A favorite of mine are a set of 4 dice with obviously strangely number sides. Bradley Efron [1] invented a way of numbering four dice so that no matter which die is chosen by your opponent first, there is a die that you can pick from the remaining three that will roll a higher value that your opponent two-thirds of the time!!!
They can be weighted so that one face lands down more than others. One approach to detecting such weighted dice is to burn them looking for metal residue in the ashes. Cheaters got around that by simply hollowing out parts of the dice to unbalance them. Both metal weighted and hollowed out dice can be detected by dropping them in a glass of water and watching to see if they tend to turn while floating or falling through the water. Cheaters got around that common test by using mercury weighting with a chamber in the center where the mercury resides when the dice is to act fair and a connected chamber near the edge where the mercury is knocked to by the cheater rapping the dice on the table before a roll. Now, casino dice are clear to make it easier to detect weighted dice, but of course cheaters still try to put bits of heavy metal under the painted dots!
Dice can also me made in a fashion so that the tend to roll off some sides more easily. This, naturally, is more of a problem with dice that have beveled corners. Casino dice have nice sharp edges.
Instead of being rounded, some sides of the dice can be cupped, causing a suction cup effect that causes the dice to roll less easily off of one side.
Dice can be manipulated by narrowing them in one or more dimensions. Non-cubic dice may have just slightly smaller faces on some numbers or slightly slanted faces, making them roll in a non-fair fashion.
In the army, dice games are often played on the surface of a bed's blanket. Cheater put a small little burr below the surface in one of the dots to make it catch on the blanket fabric.
Finally, there are "joke store" dice with bad numbering, for example no single dot face.
A favorite of mine are a set of 4 dice with obviously strangely number sides. Bradley Efron [1] invented a way of numbering four dice so that no matter which die is chosen by your opponent first, there is a die that you can pick from the remaining three that will roll a higher value that your opponent two-thirds of the time!!!
[1] https://en.wikipedia.org/wiki/Bradley_Efron