So can I just throw it 300+ and if I don't have 0.166666% each result (1,2,3,4,5,6) it's not good ?
I don't think there is a way to measure entropy of the generator from the outcomes. Also, when somebody says that something has "N bits of entropy", they are assuming an ideal RNG.
As for measuring the quality of your dice rolling, I think that simply measuring the uniformity of the distribution for a large number of rolls is probably sufficient, since a roll is probably not significantly affected by a previous roll or the time of the roll or the conditions during the roll.
If you want to be more thorough, then here is some information about other tests you can run:
https://ws680.nist.gov/publication/get_pdf.cfm?pub_id=906762Dice rolls are never random.
If you knew all necessary information (exact surface conditions, air resistance, rotating speed, ..) you could predict each roll with your dice. That's far away from being 'truly random'.
That's an extreme statement. You could say the same thing about Brownian motion. In the end, it doesn't really matter if it is truly random or not. The end justifies the means when it comes to an RNG.