If it is a constant chord wing, the c.g. is usually about 1/4 of the chord from the leading edge. This works well also for a tapered trailing edge if the leading edge has no sweep back. Use the longest chord line, usually next to the fuselage. If the leading edge is swept back (or forward), that's a whole different situation and you probably don't want to know. If you do, I'll send you a web sight that explains the formula (I can't remember it right now and I'd have to do a little research. Seems like it's on the AMA web site but I'm not sure).