Look at the shock mount ,jus ahead of the rear wheel if it's leaking you'll see oil on it and the linkage, of course if he's powerwashed it it may be clean & no oil in the shock. Best thing is to let the air out of the shock a and go for a ride, find some hard dip or bumps, if it bottoms out hard there's a good chance it's shot. Not a deal breaker if you can get the bike at a reduced price, $500 will buy an aftermarket shock of your choice.