2G had a good comment about marking the spar (to aid with dihedral alignment). After you have successfully assembled the wing and the pins are in place, take a black "Sharpie" pen and scribe a line against the back fuselage to indicate proper rear spar dihedral alignment (right wing), then scribe a line along the intersection of the front and rear spars (on the face of the rear spar) to show proper dihedral alignment of the front spar (left wing) relative to the rear spar. After practice, you will get 98% alignment by just visual cues, then the pap smear technique will get you that last 2%.