How to arrive Centre of gravity point in overall vehicle (Tractor/Car) layout

How to arrive Centre of gravity point in overall vehicle (Tractor/Car) layout.

Available data are

1. Wheel base/Wheel Track

2. Tyre Details

3. Over all vehicle layout cad assembly/parts (In virtual only. Not physical)

4. Overall weight of the vehicle

Front and Rear reactions are not available.

I am using CREO-4 parametric. 

Kindly suggest to find the COG point in virtual layout assembly.