Correctly balancing your rc airplane is so important for safe flying, because an incorrect Centre of Gravity (CG or CoG) can potentially result in the plane being quite uncontrollable.

As a very general rule of thumb the CG will be about one-quarter to one-third of the wing chord (width) back from the leading edge of the wing. The main spar, if there is one, often lies in this general area.
Again, this position is only a generalisation and in reality a CG point can be found anywhere from, say, 25% to 50% of the wing chord back from the leading edge. A CG point outside of that range is rare, but not impossible.

A quick and easy method of balancing rc planes is this, if you don’t have a special plane balancing tool: Place the tips of your index or middle fingers under each wing, exactly on the line of the CG (i.e. the specified distance back from the leading edge of the wing or nose of the plane) and a couple of inches out from the fuselage sides. Gently lift the plane up so it is clear of any surface and let it rest freely on your fingertips.