How to find CG of Sig model airplane..??

Download the manual and find out what they spec

If the wing on your airplane is basically straight (and not swept), and if it is conventional wing first, and not a canard.. you can basically measure the chord of the wing (leading edge to trailing edge), and multiply times 25%. One forth of the way back on the wing. Just about every standard airplane will balance between 25%-30%. If you opt for nose heavy, say 20% back from the leading edge you may need slightly more elevator input. If you move the balance point rearward, you will need a larger bag to put the pieces in. Rearward CG location.. a little at a time, will make a airplane less and less stable. Might be great if you want snap rolls to be more crisp. But if you go too far, the airplane becomes uncontrollable. So if you don't have plans, start with slightly nose heavy, and move it rearward as you become more confident, with good control.

If your model has a lifting tail (top of the horizontal stabilizer is curved, bottom side is flat).. the CG can go to the rear a bit more.

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.

Great pic