Best thing to do is just fly inverted. You should need a little bit of pressure - not so much a push - of down elevator to keep it flying inverted w/o losing altitude. As mentioned, the CG location in the manual is a starting point. Fly the plane and see what works.

Second test is fly straight and level, and roll into a 90* banked turn. If the tail drops, or the nose suddenly drops, you know you are tail or nose heavy.

These are generally the first things mentioned on trimming charts. You have to get the CG right before you start trimming other parts of the plane, as CG will affect pitching to the gear or canopy in knife edges, etc.