The mobility of wearable computers makes usability testing difficult. In order to fully understand how a user interacts with the wearable, the researcher must examine both the user's direct interactions with the computer, as well as the external context the user perceives during their interaction. We present a tool that augments a wearable computer with additional hardware and software to capture the information needed to perform a usability study in the field under realistic conditions. We examine the challenges in doing the capture and present our implementation. We also describe VizWear, a tool for examining the captured data. Finally, we present our experiences using the system for a sample user study.