How to check the balance of your D20

So I have a friend that believes his bad rolls while playing Dungeons and Dragons was his luck just soured. So I wanted to show him that it isn't luck but the dice he was rolling with. I decided to come up with this test. Using an old golf trick you can see if your dice are decently balanced or completely off. If you have that one D20 that rolls low now you can see why! This is the receipe for my current solution: 1/4 cup of hot tap water (our water is a little hard) 6 tablespoons of Espon Salt 1. I p