A diagnostic gage (12) that can be implemented into a tissue-simulating headform (17) or other anthropomorphic surrogate test device (11) as a means of determining the internal strain within the test surrogate. One embodiment of the gage consists of a matrix or substrate embedded with x-ray contrast agents (14) and a series of holes within the substrate (15) that provide contrasting markers in an x-ray image and a means of closely coupling the gage to the test specimen. The relative motion of these contrasting markers can be monitored using x-ray fluoroscopy equipment (e.g., source (10) and detector (13)). This gage provides a means of determining the internal strain within a headform surrogate model for the purpose of evaluating the performance of helmets in terms of reducing the occurrence of concussion among other biomechanical injuries from trauma.