They should be insured to drive the vehicle so insurance is down to them, if they cause any damage to your vehicle, they are still liable to pay out of their own pocket as most insurance when driving other vehicles is 3rd party only.
I would check two things before letting them sit in the driving seat, 1 they have an insurance certificate that says they are covered to drive other vehicles with the owners permission, 2 they have the money to pay for the vehicle if they write it off.
People test drive cars with no intension of buying them so they might not have the money if the worst happens so double check, and if your still worried type out a little disclaimer and get them to sign it.