That was good thinking Jasmin. You are quite correct. You would not even need to throw the dice millions of times.
The way we find out such things, we call statistical analysis, and by carefully counting what we get when we throw the dice, we probably could tell whether the dice are fair or not after just a few hundred throws.
It is in fact very difficult to design dice to be exactly fair. The slightest difference in the rounding of the corners or the lengths of the sides can make a big difference to the way the dice roll, and the score that you get after some hundreds of throws.
As for you, just keep thinking about such things. The more you do, the more interesting the new things that you might discover.