To evaluate the purity of bullion, you can use an electronic tester, which uses conductivity to determine the fineness of the metal. Alternatively, you can perform an acid test, which involves applying a drop of acid to the bullion to test its purity. The acid will react differently to different levels of purity, allowing you to determine the purity of the bullion. It is important to note that an acid test may leave a small mark on the bullion, which can reduce its value.