A guided wave based reference-free non-destructive testing (NDT) technique, which does not require direct comparison with previously obtained baseline data for defect detection, has been recently developed and applied to simple plate-like structures by the authors' research group. In this study, the reference-free NDT technique is further advanced so that this technique can also be applied to specimens with complex structural geometries such as holes, stiffeners, and varying thickness. A unique damage-sensitive feature for each structural configuration has been extracted, and a suite of damage detection algorithms, which is applicable to all the investigated structural geometries, has been proposed. Experiments as well as numerical simulations are performed to investigate the applicability of the proposed NDT technique to structures with complex geometries.