I am very new to raising chickens. Our neighbor gave us a few older hens. 21 to be exact. How do I know which ones are laying? Something about their pelvic bone, but not sure? Any help???

Some very general rules of thumb:
A laying hen will have a bright red comb, almost waxy looking. A non-layer will have more of a pink comb. You can catch them and turn them over to check their vent (this is where the egg comes out). A layer will have a moist vent. You can feel the bones on either side of the vent opening and a layer (of a standard size breed) will have 2-3 finger width span between those bones. So you just lay your fingers between the bones and check that way.

Do you know how old these hens are? It is possible they are all laying but as they age their laying frequency decreases.