It's a good idea to shake the toner lightly before installing it but you'll never hear a sound like pebbles if the toner is bad. If we do, then Xerox has even bigger problems than we all thought. The bad toner will have toner particles that are minutely larger and they cause an arc at the drum that leaves those nasty spots. Not only will you not hear them rattle but you couldn't even see them if you opened up the toner.
So unfortunately we don't have a way of knowing if it's bad toner. Xerox never publicly shared any defective lot/date numbers. Good thing they don't sell chicken, we could all be dead by now.
My last few deliveries of 17x11 Tango 10 pt. C1S had random handfuls of the stock upside down (coated side facing down instead of up in the box). Nothing consistent. Given the way stock is sheeted I can't figure out how this would happen.