The questions on the diagnostic exam are meant to be a guide and test on what you have already learnt. As the poster above mentioned its very easy to repeat the exams and eventually your doing the same questions over and over.
Eventually, your going to ace the diagnostic and will end up with a false sense of hope that your knowledge is good enough to take the exam.
The best way to know your ready for the exam is to go through the Syllabus guide and make sure you are comfortable with all the points (or at least most of them) that they require you to know.