Unless you have a wind tunnel wide enough to place the car sideways, it's not easy.
However, you can approximate it using a wide F stop elevation photo (side view, as little perspective as possible) of the car.
Use any photo program, and color in the entire silhouette into a solid (wing sideplates, roof fins, hood scoop) primary color with no outline.
Now convert it to a .bmp, and use a little program I found (e-mail me for a copy).
Using the program, open the .bmp, and select the color, and it maps the color field and locates the AC and CG (but this is useless since weight not related to area).
The location will be at a proportion to the length in pixels, so just crank in your total length in inches blah to get the real AC on the physical car.
Not exact because cannot compensate for radii, effect of open wheelwell, etc. but a good start, and you can save the file, and photo the car again with new details (bigger scoop?), and see how far the AC moved.