Are you getting them from a breeder, a show, or a hatchery? Depending on that will determine what to look for.
Quick look over: They are not super big birds, they are bantams. The necks are not super long, though some are breeding that way. Both bearded and non-bearded is accepted, but if they are bearded you shouldn't see any wattles, and very small ones if you can see them at all on the non-bearded. Ear Lobes are Turquoise or dark, not red, Comb looks like a walnut mulberry colored.
Closer inspection, look in the wings and tail feathers, see if there is "hard" feathers, or if they are all silkie. They need to be as soft and silkie as possible, though hard feathers are accepted, not preferred. Look at the toes, make sure there are five toes, with toenails on each one.
Look at the silkie breeders website, start looking at their pictures, you will quickly see the differences when you see hatchery stock after looking at national winners!