Hard to test as no core code uses this hook. It adds an extra param to the data array that is passed so hopefully the code is explanatory enough to pass without interface testing. If you really want to test this you could: 1. This quick and dirty plagiarism plugin can be used for testing purposes. I wrote it while learning a bit about plagiarism with this issue: https://github.com/stronk7/moodle-plagiarism_dummypla 2. Set up an assignment with online text submission, and perform some submission. 3. View submissions page and check that submission data is debugged out (within a green or red message from the plugin, depends if the submission is considered plagiarism or no).