zr861:

Measure the chord of the wing.
The model should balance with empty tank, but otherwise ready to fly, when sustained at a distance back from the leading edge of the wing of 0.25 times Chord.
That may be a little nose heavy, but good for a first test flight.

Then, using a trimming chart, find the best position by experimentation, paying attention to the reactions of the model.
Mark that point with something permanent.
The normal range is 25% to 33% of the wind chord; however, some models have the CG beyond that range.

Do not forget to perform a lateral balance as well.

Regards!