There are too many scenario based questions to think of them all, but use what you can come up with to see if your student can correlate basic information about systems, regs, and operating techniques to come up with a reasonable response.
Go through the PTS and hit everything as a last review. Expand upon those specific items listed, for example "preflight planning" encompasses a lot information. If the student knows the material, then this process should be fairly quick and straightforward. You should only have to clarify a few points here and there. If you find yourself having to teach/reteach very much of the material, then the student probably isn't ready. Be sure to review the introduction of the PTS as well, so the student knows exactly what is expected of him/her during the ride and what the examiner is required to be looking for.
Make sure that the student knows how to reference the FAR/AIM, POH, and other pertinent materials with some efficiency. IMHO, just knowing the speed limit below 10K is 250 because someone told you isn't good enough. The student should atleast know it is in Part 91 and should be able to find it with relative ease. (This was just a 'for instance').
Hope this helps.