It depends on the quality of the rack. If it's a strap on type rack I would either park where you can keep an eye on the bike 100% of the time, or run a lock through the frame and the bumper or towing eyelet if you have one. If you can do neither, I'd take the bike off and either put it in the car or chain it to something substantial.

If you have a rack that is securely mounted to the car, (Locking towers, locked to the hitch) then I'd just cable it through the frame and wheels and hitch. I will use multiple cables sometimes just to make it more complex. Just remember that anyone with a good bolt cutter can cut through most cables.