Measure the width of the wing (cord) from the leading to trailing edges as close to the fuselage as possible.
Take that number and divide it by 4. This will give you the standard 25% figure.Example: if the wing measures 12" then the CG figure would be 3"
Using this number,measure back from the leading edge close to the fuselage and put a mark on the bottom of the wing on each side. Typically this works out to be very close to the main spar in the wing.
Lift the plane up, put a finger or thumb on each mark and test the balance or you can use a balance machine.