If there's ever a question about a particular year a Fiero is, simply ask for the VIN. This isn't any sort of top secret information, and any seller that refuses to divulge that information is either mis-informed or incredibly ignorant.
The key is the 10th digit:
1984 - E
1985 - F
1986 - G
1987 - H
1988 - J (the letter "I" was skipped, as it looks too similar to the number "1")
1989 - would have been "K" for production Fieros had production proceeded.
Letters and numbers around it designate particular trim levels and engine options, and are for the most part similar throughout production. There are however some anomalies, as VINs often do not fully differentiate what a particular Fiero may be. For example, the 1984 Indy pace car replicas use the same VIN trim level designation as the '84 SE cars, making positive identification at a glance difficult and requiring decoding the RPO fender tag for positive identification.
I keep a little printed out piece of paper with me with all the VIN ID information (you can find it on Wiki) in case I come across a Fiero and question it's claimed appearance by the seller.
Most all of the body panels on every Fiero made swaps with little effort, other than sometimes requiring specific brackets or specific fender liners, etc. Regardless, it's relatively effortless to swap a panel out onto a year Fiero that it did not originally came with. Also with Fieros now well over 30 years old we'll likely run into many examples that have non-OEM equipped panels swapped out.
[This message has been edited by Fiero84Freak (edited 12-26-2014).]