It's easy to find out CofG spot for any ARF, or model in kit form, just get a hand on the instruction manual, most are listed online. Different story with scratch build models of your own design.
I got a simple solution, I normally build smaller, identical scale prototype either as a hand or catapult/bungee launcher to give me a good idea what the real thing will be flying like.

Well before finish of the project I'll check the CofG with smaller built proto.

Sling shooting cg trials.


Upgraded from 8 to hot 10S.

Another example.

Mini proto launch trials.