Many people wonder if they should use discriminate and when they should be digging if they hear iffy signals. While there are times to use discriminate and times where I don't dig iffy signals in general you should be doing the following until you know your area and detector very well:
Iron = Don't dig
Not Iron = Dig
Iffy signal - if its repeatable and not iron = Dig
Faint/Deep signal - check to see if iron. If not, dig. If iron tone, clear a few inches of sand/dirt and check if its iron again to confirm. The deeper the target the more trouble machines have at telling what kind of metal is beneath the coil.