I agree there is no sure fire way to tell in advance or a way to improve your odds.
When I get tired of digging foil signals I do start becoming more selective, instead of giving up completely. I look for signals that do not vary in ID#s no matter which way I sweep the coil over it. I ALSO listen for a double blip. Back when I only did the beaches, most foil signals that gave off the double blip as the coil went over it was a ring or a washer or something with a hole in it like a pull tab ring.
I hate passing foil signals up, I look for the deeper ones, but, who is to say a recent ring loss hasnt occurred?
Its part of the hobby........
Tony