A crucial aspect in the implementation of an augmented reality (AR) system is determining its accuracy. The accuracy of a system determines the applications it can be used for. The aim of our research is measuring the overall accuracy of an arbitrary AR system. Once measurements of a system are made, they can be analyzed for determining the structure and sources of errors. From the analysis it may also be possible to improve the methods used to calibrate and register the virtual to the real. This paper describes an online system for measuring the registration accuracy of optical see-through augmentation. By online, we mean that the user can measure the registration error they are experiencing while they are using the system. We overcome the difficulty of not having retinal access by having the user indicate the projection of a perceived object on a planar measurement device. Our method provides information which can be used to analyze the structure of the system error in two or three dimensions. The results of the application of our method to two monocular optical see-through AR systems are shown.