Since I don't even take my own bikes up to 100 mph, that would be a big red flag that would cause me to refuse to let them take a test ride on it. I don't see how that helps you determine fork damage. If the bike has never been down, I would just reassure them that there is no fork damage and I would buy it back if somehow they find that there is (which they won't). At 100, they might be able to find an improperly balanced front tire, though.
When I take a bike for a test ride - which I'm sure I've done more than 100 times - I want to make sure that it shifts up and down smoothly, that the throttle doesn't have any dead spots, that it generally handles well, and that the brakes work like I would expect them to. I can learn all I need to in about five minutes, unless I'm trying to figure out whether or not I like that particular bike because I've never ridden one before.
After I wrecked my 1997 Ducati Monster and bought the ST2, I was still looking at Monsters and took at least 8 of them for test rides. Most of them were just missing something that mine had - engine powerband, brakes, other general behavior. One, which was a salvage bike, was actually dangerous. I felt lucky to make it around the block on that thing without crashing it. I probably wouldn't have taken that one for free. It was after riding one in Livermore that only had corrosion damage from being left outside too much, but was otherwise excellent, that I decided that I really didn't want another Monster and stopped looking.
I've done some looking at various XR650L's too and even bought a second one a couple of years ago. Several of them had some sort of "miss" in the engine that might have been due to improper carburetion or something. If the bike doesn't feel quite right AND the seller wants top dollar, I usually pass. The one that I bought had the jetting/miss issue, but the price was right and rejetting fixed most of it. Selling it to OaklandF4i got it to the "running perfect" point. He rejected my offer to buy it back. :laughing