There are a number of things they can look for, but the first is tiny scratches on the flat areas of the facets ... the fakes, including moissanite derived gems, scratch easily as compared to a real diamond. Additionally,if the diamond is un-set... that is loose, place a sheet of white paper under a direct light source and using a lead pencil, draw a black dot about the size of a large period. Make it quite black. Place the diamond or supected diamond face down on the dot. If it's a real diamond, the dot will not be visible since the light is defracted so much, but a fake diamond will display the dot clearly...