How to distinguish fake and real Hamon?

Before answering this question, you have to figure out what is the Hamon. Hamon is that wavy line on the cutting edge of a katana. Traditionally, this wavy effect is achieved by differential hardening. The differentiation that is seen is actually the result of a gradation resulting from the heating and quenching of the blade and its geometry. This is the process where the blade is quenched so that the edge is harder than the spine or body of the blade. The spine of the blade is coated with a clay mixture, then heated and quenched. The thick clay coating on the spine acts like an insulator and causes the coated portion of the blade to cool more slowly (the slower the cooling, the softer the steel).