you always need to check a driver has insurance before letting them drive your car. If they have no insurance then you could be prosecuted for allowing an uninsured driver to be in charge of your car - whether there is an accident or not.
If thier insurance policy allows them to drive another car not owned by them, and you have insurance active on the car then you're fine.
If they have taken out temporary cover on the car then you're fine.
If they don't have the right cover, then they simply can't test drive the car on a public road.
[Eight pot is correct I found this out the hard way!