I had a car that had several mods declared and when I crashed it (no one else involved), the repairer had to sit on it until an engineer (not an assessor) went out to inspect it. He later told me that the bloke had come to check for other mods because declaring the few I had, they thought I'd have more. I didn't and they paid up which was good.
Mods were - airbox, filter, dv, exhaust, disks, brake hoses, strut brace, lowered.
I think they were looking for cosmetic as typically someone who's done the other work will have added wheels, lips, skirts etc.
They also would only have paid out OE replacements.