vehicle presence sensor
Parking garage sensors use ultrasonic distance measurement to check for something that is closer than the floor or IR reflection as far as I have deduced from the look of the roof mounted sensors. I suppose micro power impulse radar or a inductive metal detector system could also be used.

Much depends on how cheap and how accurate you need the sensing to be.

A camera and some clever software is a practical option but will be expensive if you have lots of parking bays.  

An optical mouse camera may be the cheapest novel option for large numbers and can measure the speed of the car entering the bay if it is located above the parking bay.  Some can be configured to output the 16x16 pixel image in debug modes. Remember you heard about it on electronics stack exchange first :-)

EDIT:
If you can put a bar-code on the roof of the car you could scan then as they drive into the yard or parking bay to make sure it it the right car.  Use IR absorbing strips on the roof and they will not be very visible.

Image recognition on a numberplate could work, you need to take a photo every 30 seconds and see if there is a valid plate.