I think about the only thing you can be 100% sure of, with any detector, is that when it beeps, there is something metal down there. The rest is just a calculated guess, to help you decide whether it's worth the effort to dig it up. You will never really know, until you recover that piece of metal. Nothing wrong calling it trash, and passing it up, because you'll never know any different anyway...
I don't mind digging a lot of holes in my own yard (or my brother's), but more selective out in public. I want to be pretty sure it something good, if targets are just a foot or two apart, not looking to plow up the turf. I can always go back later on, after some recovery time.