If you know anyone with a GPS receiver, it's easy.
On my old Pathfinder, I used the GPS receiver's Speed display, and made notes of the actual speed when the speedodometer read 30, 40, etc.
For the summer tires I used, the speedometer was high by about 3%, but for the smaller sized winter tires, it was high by 10%.
On my 06 X OR with the stock tires, the difference between the speedometer and the GPS-measured speed is not even worth bothering about.
I did my GPS speed measurements on flat roads, but it will be 'close enough' even on hills. I also once asked a police officer who was setup with laser radar to check my speed, and his result was the same as my GPS.