How do I test the protein purity?

The most common method for testing protein purity is gel electrophoresis. In general, the method is performed in gels composed of polyacrylamide polymers, which act as molecular sieves slowing the migration of proteins approximately in proportion to their mass-to-charge ratio. The migration of the proteins in the gel during the electrophoresis is a function of the size and shape of the protein. Gel electrophoresis uses the detergent SDS (sodium dodecyl sulfate). The presence of SDS causes electrophoresis to separate proteins almost exclusively by their mass, with smaller peptides migrating faster.