Go into the question bank, and perform a number of operations, and make sure there are no notices or other PHP errors. When I say a number of operations, I mean: Create a question, Edit a question, Move some questions to another category, Create a category, Edit a category, Export some questions, Import some questions. If you want to make it a really stringent test, create a role with a restricted set of question permissions (e.g. use, useall, view, viewall and editmine) and then repeat the tests you can with the custom role.