Many things of the GT group are easy to add. Dual exhaust, with quad tips and rear valance with the cut outs. Heavy duty suspension which gave stiffer spirngs which is nearly impossible to discern without disassembly. Heavy duty sway bar of 1 inch diameter.
If you can get the car up in the air and inspect the exhaust system and rear valance it may shed some light. The stock system will have matching left and right exhaust hangers just in front of the rear end. They should attach by two bolts that go up into "capture" nuts in a bulge in the floor pan up about a foot from the bottom of the floor pans. Not a tell all, but if the exhaust system is completely custom, then it may have been added. Custom exhaust hangers normally are attached by easy to notice bolts right through the turn of the floor pan behind the back of the back seat.
Second if you can inspect the rear valance and find a date stamping on it, it should be within a few days of the date stamps on the rear quarter panels and front fenders. If it has not lost those also. At the same time I have seen cars with date codes that are a couple months apart.
But if you see a custom exhaust and a repop or new rear valance, you might ask a question or two.
A Marti report is very cheap and also will state if it has the GT group.
Rob