How to test for good passivation/phosphating?

I wondering whether there is any test for testing the passivation on the surface of the metal. Normally Salt Spray Test has been carried out for testing coating on the surface of the metal, however when I asked my supplier who carry out passivation for our product, they replied that salt spray test is not the correct test to perform for passivated surface. So may I request you whether can you recommend any test for passivated metal surafce