A good link from Old Fogducker. Wingspars are fairly easy to see; the next time you're in a hangar and there's an aircraft on maintenance, ask one of the engineers to show you. Even on something as simple as a PA28, there are at least three spar inspection access panels per wing and with the wingtips off you'll get a good view of the rib structure too. Lift up the rear seat of the Cherokee to see how the wings are held on: the spar sections are at their thickest there.