in order to measure the accuracy of a XPS you need some cases which function as "gold standard", i.e. cases for which the solution is known, but which where not yet used for the development of the system (that is also some "good evalution practice" in machine learning), otherwise any evaluation of the accuracy would be biased.
Measuring the accuracy then means to run the XPS on those test cases, record the result and compare it with the expected result for building up a confusion matrix. For details of evaluation of this confusion matrix and start learning about evaluation measures, you can use Wikipedia and search for "accuracy", "recall", "precision" or "F-measures". The later three are usually prefered over accuracy, since they carry more information about "false positives" and "false negatives".