These aren't the best pictures but hopefully they will help. since your plane is covered, you might not be able to see the edge of the sheeting clearly. In that case, just feel for it.

Basically, the spars are the "beams" that run the length of the wing and provide strength. The leading edge sheeting forms the front section of the wing on both top and bottom and usually stops at the spar.

Once you find the spot, measure back 1/8 and make a mark. That's where the plane should balance.