For quite a few years I hunted with a machine that didn't have a meter. When I would find a plate it would just be a very strong, solid reading. With the front being brass it would really ring out. But if a plate is laying on it's side, the reading will still be solid, but more like finding a bullet or a coin. Then, when a plate happens to be very deep, it will usually ring out like a bullet. All this just means, there really isn't a way to identify the target as a plate. And then remember, different machines will have a slightly different tone and the meter readings would vary from machine to machine. So it still gets back to just dig all good targets and hope the relic gods are smiling on you.