Thin film delamination and sub-surface cracking represent important reliability issues for the semiconductor and thin film magnetic hard disk industries. Detecting such defects is often difficult and requires complicated specimen cross-sectioning and imaging. Because the sample preparation process itself may cause crack growth, such a method may be used only as a qualitative tool for defect characterization. In the present work, we introduce an in-situ non-destructive technique for detecting sub-surface cracks by continuously monitoring the contact stiffness while dragging a probe across a smooth surface at an ultra light load. As the probe encounters a sub-surface crack, the local contact stiffness decreases dramatically. The amount of reduction in contact stiffness depends on the crack size and geometry and the position of the crack relative to the surface. Results of the scanning stiffness microscopy experiments conducted on hard carbon and chromium thin films are described and discussed.