be anything that tests the laws of physics. some popular reality tests include pushing your finger against the palm of your hand or doing a small jump.
as you probably know, a lot of the content of your dream is made up from the things you have been doing the previous day so the idea with the reality check is that it will become a habit and, eventually you will do the test while you actually are dreaming then, of course, when you do, the laws of physics will bend, i.e, your finger will go through your hand or you will jump higher than normal or hover or even fly, and 