A common way to check the CG is by flying inverted. The plane should generally require a little downward pressure (push) on the elevator. The nose of the plane should drop a little. Of course then there's tweaking out to fit the style of flying you want to do. Some 3D guys have the CG so far back that they don't need any elevator for inverted flight, or if you want very stable plane and don't fly inverted a lot you might want a forward CG.

Other things to do are to roll into a vertically banked turn and see if the nose drops. Also see what happens in knife edge flight. There are trim charts that outline all this, but it starts with the inverted flight routine. Can also be thrust angles, etc.