First, make sure they’re who they say they – and that they can drive – by checking their driving licence and taking a note of their details. Secondly, make sure they’re insured to drive your car; and, if they’re not covered by your insurance, insist on seeing proof that their insurance does cover them.
When you’re on a test drive, let the buyer choose where they go, as they may well be suspicious if you insist on a particular route.
What you must insist on, though, is that the buyer is never left to test drive the car on their own. At the very least, you should accompany them; and, if you want, take someone else along with you for extra security.