Recognizing good rock takes a lot of practice. About the simplest test or method of evaluation I can suggest is to knock a corner off the piece and see how it flakes or crushes. If it flakes, it may be workable. The smoother the surface left by the flake detachment, the better the stone. When you knock a flake off, if the new surface is grainy, most likely it is not too friendly to knapping. If it is smooth, it probably has potential. Beyond that, some rock seems to flake OK, but takes a tremendous amount of force to pop the flakes off, or the flakes come off leaving a gritty surface. Some such rocks need to be cooked to become primo material. There is a lot to evaluating rock.
WA