If you've been playing D&D for a while you've probably come across a couple of dice that always seem to roll better or worse than others. In some cases it is obvious that the dice are malformed and will therefore not be random but in others it can be harder to be sure.
Enter the salt water test. Essentially what you do is take a cup of warm water, drop the dice in and then add salt until the dice float. For light acrylic dice this will take almost no salt but for denser resin dice you will need to add quite a bit of salt. For many denser resin dice it is actually impossible to get them to float in salt water as the water reaches saturation (no more salt will dissolve) before the dice will float.
Once you have the dice floating you just need to poke them a few times and see if the same number keeps floating to the top. If the dice come up in an obvious pattern here they have a density issue (probably an inclusion or void) and probably shouldn't be used.
It should be noted that most modern dice will do fairly well in this test but it isn't definitive proof that the dice are random. To actually get confident that the dice are truly random you would need to roll them a few thousand times and analyse the results. Likely you don't have time for this so:
What should I look for to get truly random dice?