That sort of doesnt matter, IF the car has "full self driving already enabled" as long as it was paid for and not just some mistake of accounting.
If the car has FSD but no hardware 3, OP could schedule an appointment for free HW 3 install, so it doesnt matter if it has it now really ... again thats as long as It is a purchased FSD and not something that has been left on by tesla by mistake.
Other than looking at the exact same thing you would look at in every other used car, it might be helpful to know what a 100% charge is NOW on the car. If you are paying a premium for a car because it has FSD, make sure it actually has FSD purchased on it. the rest would be the same thing you would look for on every other used car.
You dont mention there were a few different ones available, including a couple which are not available now (Long range RWD, and Limited edition Mid Range).