Only way to know for sure is dig it up!
Generally speaking, with Tesoro machines, if you can get an audio in one back-and-forth direction but not the other, it usually means is a long and thin item (nail), and iron. That has been my experience, but you need to be confident of your machine and what it is telling you, so "guess" your target is iron by the sound, then dig it. When you get to the point that you are right 100% of the, you can stop digging those targets. That said, I occasssionly dig iron but only when its deep and iffy. I would rather dig a questionable one and get iron, than walk away from a potentially good target, especialy if I am on an old site.