If you really want to be sure your het is in fact a het buy from a reputable breeder and/or make sure to get paperwork of the genetics of the animal your buying. Other that that there are really no consistent ways to tell a het from another normal.
People come up with traits they see a lot that usually ends up with the animal being a het (ie. Het pied marker/tail stripe) but they are not proven or consistent as clearly known 'normals' sometimes have the markers as well....so basicaly get your paperwork .