woodyww, are you talking about crown caps ? Or aluminum screw caps ?
If you were referring to crown cap type, try this: Next time you suspect a crown cap, play with the swing speed over the suspected target. Momentarily speed up the swing. I believe you will see that if they are crown caps (versus penny or dime), they will start to "break up". Versus if they are a coin, they remain the same sound or get better.
This was ESPECIALLY pronounced on the fast-swing machines of yesteryear (6000d, 6db, etc...), but .... is still there a teensy bit on new machines. Crown caps behave differently with speed-of-clip.