What does it weigh? I have a thought, but your vehicle may crush the setup/measuring tool.

Here's one option:

http://www.wikihow.com/Calculate-Center-of-Gravity

Or, my thought (With a stronger version design) would be:



This is what the RC plane guys use. I figure you can do the same with some metal rods and a 2x4, only put the metal rods closer together than what's above. Having two rods stabilizes the weight from side to side. Then you just put your truck on top of the two rod ends that are sticking up, and move the setup front to back on the bottom of the truck until you find a spot where the truck is pretty close to be exactly balanced on the rod ends. Where the rod ends are located at this point is where the center of gravity is.

The setups are very easy to make. Simply drill holes in your wood piece that are 1/1000" or 2/1000" smaller than the diameter of the rods that you'll be using. Measure out the distance between the holes, so that the rod ends will fit nicely in the desired location of measuring point within the chassis (Or whatever) BEFORE drilling. Be sure to drill the holes the same depth (Or all the way through, and glue them flush with the bottom of the board) or the ends of the rods won't be the same height when in there. Put a dab of all purpose glue (Or glue that sticks to metal and wood. (Or whatever materials that your using.) in the holes. Use a small rubber mallet or the like and pound the rods into the holes until they bottom out. (Or go all the way through.) Let them dry. (Be sure that if you did the 'drill all the way through procedure', that the rod ends that are in the holes in the wood are flush with the bottom of the wood board.) Simple as that!

I realize these are detailed instructions (and way too easy for you to handle), but if someone else is wondering how, then this will be a good reference....

I do recommend using something wider than a 2x4, because with the weight of a 1/6 scale, it may not be very stable while your trying to measure it.