Simple, cheap approach:

Place the index finger of each hand on each side of the fuselage with the plane facing forwards (you are looking at the tail of the plane). Your two fingers will act as a "pivot" point along the fuselage. Start behind the trailing edge of the wing.

Initially, the nose will be heavier than the tail and the nose will point down. Move your fingers/pivot point forward along the fuselage towards the nose until the plane balances evenly/level between nose and tail.

Where your fingers are is the CG. It is usually measured from the nose of the plane or some other reference point specified by the plane manufacturer.

If your CG is not correct, try moving the battery in the appropriate direction in small increments until the plane CG is in the proper location. If this fails to correct it, you may have to move the servos and/or other components also (depending on the plane and gear).

Best of luck.

Tom