Although the DHC-6 does not have through spars in the wings it does have spars, two in each wing – main and secondary (rear). As ‘captain2captain’ rightly says, these are where the wing/fuselage/strut attachment points are, plus of the attachments for each engine.
With any high-time aircraft it’s essential to ensure that all applicable AD’s and structure-related SB’s are complied with. The MM will detail any supplementary maintenance for the wing spares plus an aging-aircraft structural integrity programme (or equivalent). As far as practical, insist that this is carried out – particularly if you’re taking the aircraft to a remote area (where most Twin Otters are to be found). If you have any lingering doubts, contact the manufacturers product support organization for advice.