You could measure the wing chord - the distance from the LE of the wing to the TE of the ailerons. Then, multiply that number by .25 (25%) and balance the model that distance (25%) back from the leading edge. 25% is a safe, conservative starting balance point that should not get you into any trouble. The model will probably be slightly nose-heavy, but it certainly won't reach out and "bite" you so you will have an easy, safe first flight. Then, if you want, you can shift the C.G. aft in 1/4" increments (assuming your control throws are set according the the instructions, or some reasonable amount) until it flies the way you like.

If the model is nose-heavy, or if your elevator throw is not enough, the model may not flair for landing. I could go on...but 25% is a safe start. [8D]