When you design your own robot, you have to take care of dynamics of your robot and you have to specify the inertia matrix and COG (center of gravity). There are many sophisticated tools out there but you can easily do that with meshlab.
Just open your CAD file in meshlab, then go to:
Filters > Quality Measure and Computations > Compute Geometric Measures
you can find this info at the info panel at the right bottom.