The "best" way to do it is in the air.

Overall, for good 3D, it will be an aft CG. Hover the plane, good 3D balance is when in a hover, you do not have to work the elevator, normally pulling up elevator to keep the nose up. This is kinda difficult to discribe.

Another good way, is, when inverted, you want the plane to climb a bit.

Too tail heavy is when the plane balloons on landing and you need down elevator to land.

Start safe and move it back. Good flying 3D planes will fly very tail heavy. Bad flying 3D planes will snap.