Well, it depends. If I have tested the dice and I've verified that, on average, all of the values have a ~16.66% chance to return, then it's safe to assume it's
random. At least, that's the definition according to Wikipedia:
In common parlance, randomness is the apparent or actual lack of pattern or predictability in events.
I know it sounds paranoid, but I don't know what to answer if one ever made me such question. Like, sure, most of the hardware is very unlikely to have this kind of weakness, but what about hardware wallets? They're designed for solely transacting bitcoins and their random number generator, as in all hardware, isn't
apparent.
Couldn't one of all these hardware wallet companies implement something like that and steal millions of dollars worth of BTC, in just one moment? It would probably be the worst scandal of the crypto space.
In the case of dice rolls, you can be sure the number you got was random only because you were generating it yourself without trusting anybody. You have to be a witness of the whole process to verify randomness. But if you come and tell me that a certain number was generated randomly, I have no way to verify your claims. In the case of hardware, it is simply impossible to follow the whole process of generation, so you can't be a witness by definition. The best you can do is test numbers for pseudorandomness by generating billions of numbers and trying to find a pattern.