CG or center of gravity is the balancing point that gives the plane a stable flying. Correct CG on rc plane makes it easy to fly and to control in the air. To indicate a proper CG how to rc plane will help you find out from a very basic approach to a serious calculation one. A very basic CG marking can be found by using your index finger tip placed under the main wing. The probable CG will be around 1/3 section of the distant from leading edge of the main wing.
If the plane is well balanced from this measurement it is acceptable that your plane is under correct CG. How about a bit more serious technique for someone who seeks for better solution? Yes you can do that with some calculation. Take some paper, ruler and pencil then makes it straight away.
From the picture A is the length of tip chord, B is the maximum thickness of root chord, C is the length of root chord, and D is the maximum thickness of tip chord. A crossing point above will be used to determine the plane CG. How about one technique with a bit more elaborated.
This technique is also a good way which uses MAC (Mean Aerodynamic Chord) to determine the CG. The different between these two technique is one use Thickness of tip and root chord but another one use MAC value in the calculation. You can try both but will find that the result is not different in significant value. Please remember that without correct CG you will have trouble in flight.
Plane with tail heavy is most dangerous to fly. It makes bad stall and very difficult to control in the air. In contrary if the plane is nose heavy it is still acceptable to fly since the pilot can use trim control on the transmitter to counter the nose stall. Nonetheless after maiden flight the pilot must adjust the gadget inside the plane to get correct the balance of CG. From this point I hope you can find CG by yourself but if you need any help please leave me a question at the comment section down below.