Large iron will create "wrap around" effect where it will change from low tone to a very high tone/ ID number, more than often higher than what most large silvers coins would come in at (97,98,99 for me on 0-99 scale). If you run some iron audio you should hear some initial low toning as the coil approaches the iron target, before it quickly jumps to a high tone, so that can be a bit of a giveaway. If using some iron discrimination, all you might hear is the high toning and none of the initial low iron tone, so sometimes too much discrimination might only provide part of the picture on what is happening below the ground.
Iron can sound obnoxiously loud compared to say a more mellow tone of a coin target, same when pinpointing the audio may show a target with a wider cross section on the iron compared to that of a coin. If you pinpoint a suspect target and then dig a plug and find the target has moved, it may also be iron. It is just one of those things that you eventually get used to, more so when using the audio to decipher between a good/bad target vs relying on target ID's.
Digging iron isn't always a bad thing as there are plenty of interesting iron relics to be found, not to mention to help unmask good targets by removing iron from the equation. Iron is also an excellent indication of previous habitation, I often follow the iron trail to locate older areas of interest.