You can't, unless you have a dodo scale at home.  Unless you're asking how to find the manufacturer's marked cg.  If that's the case, different manufacturers use different marking methods.  Ebonite uses a bullseye, Brunswick a small punched dot (can be hard to locate), etc.  Look on the box the ball came in for the pin-out distance, then look for a mark of some type at that distance from the pin.  --  JohnP