The problem with small iron masking and falsing is that even with pinpoint it is hard to tell an iffy signal because pinpoint will tell you it is a small target, though you may get a better idea if you have multiple small targets under the coil. Falsing big iron is pretty obvious when you go to pp mode, though because you can "see" you are passing your coil over a big target so it is obviously not a coin, although what is under that big iron, so you have to dig it. I do think High VDI's pinpoint advice is about the best you can do to make an educated guess, though, without digging. Other tools to "interrogate the target" are going to AM to see if the iron tone and high tone come in together indicating a ferrous target vs. some delay between the two indicating a possible masked non-ferrous target. My other "tool" is to put the "opposite" program into the user profile slot. For example, hunt with field 2 and then put park 1 (no iron bias though) and with a diff recovery speed in the user profile slot to see if it hits differently in the suspect target and "brings out" the masked ferrous target, kind of a variation on hunter_46356's recommendation above. But frankly, but the situation is unfortunate though and the only real way to know is to dig all the iffys. If you want to be conscientious to the landowner though and minimize the plugs, there is really no magic bullet in this situation. HTH.
P.S., Another thing you might want to experiment with is Gold Mode. It sounds completely different than the other modes because it's VCO pitch vice discrete tones so you don't get a lot of audio info other than getting a good idea about the size and or depth of the mystery targets under your coil kind of like you are running the pinpointer, but it is a motion mode (unlike the pinpointer) in that you have to swing the coil to get the sound. But also unlike the pinpointer, it will give you a VDI. I have tried it in thick iron situations and found it to be good in some ways but I also missed the tone audio information. Anyway, worth a shot.