Examine your clay underneath a microscope. Pure bentonite clay has small crystals that contain traces of metals such as aluminum. The crystals are flat and squared, with several layers lying on top of the first. Metals get caught in between the crystals, giving it a shiny appearance.
Measure the amount of sand or sediment in your clay sample. The more sand or sediment you find, the less pure your bentonite is. While there will be traces of these elements in any sample, you only want to find small amounts.
Add a small amount of water to your clay. Bentonite clay will swell and become extremely pliable when mixed with water. When the clay is impure, it will not possess the flexibility pure clay has. It will also fail to swell as it should if it is impure.
Feel the texture of your clay when dry. Bentonite clay will feel like powder in pure form. Impure clay will be rough and coarse like fine gravel particles.