Silkies often have this defect. This can be a visible defect or only seen when tested. The visible defect is a wing not folded and held close to the bird's body. Either the primaries jut away from the body and the rest of the folded wing or the bird holds her wing low.
Take care not to confuse worn or near molt feathers with weak wing. The defect sometimes only shows up when you test for it. Gently and fully extend your bird's wing. Release the wing. Your bird should rather quickly snap her wing toward her body and neatly fold it. If her wing only slowly returns or doesn't neatly fold, that probably is a weak wing.
The primary and secondary wing feathers do not form a smooth arc when a bird has split wing. The primaries and secondaries form their own arcs. The arc meet where the primaries and the secondaries meet. There is an indentation because the last primareis and first secondaries are too short. Be carefull not to confuse feathers still growing with split wing. For a while, until Alice corrected me, I would stretch out the birds wing unnaturally and too far. Of course that shows a gap between the primary and secondary feathers. It is not split wing.
Over Folded Wing
Some Silkies fold their wings too high or thier wings reach too close to their tail. I call that an over folded wing.
Twisted Wing Feathers
All the primary and secondary wing feathers should lie flat in the plane of the wing. Of course they are slightly angled to tuck under their neighbors. Any twist beyond that is a twisted wing feathers. As are many defects, this defect can be either genetic or environmental. Sometimes a molt will cure twisted wing feathers, if it doesn't, the cause is probably genetic.