I am thinking of doing the PADI divemaster course. Before I go and sign up for the course, what questions should I be asking the shop providing the course?
As several replies have indicated, you should ask about the instructional format (for theory and pool skills), and about the approach to the experiential portion. To that, I would add questions about what is expected of the DMC in the experiential portion, whether you will be a solo DM candidate or part of a class, and what the usual time to completion of DM training has been in the particular environment in which you will be training. Some environments approach DM theory as a self-study course - you do the reading and we administer the exams, and if you pass, fine. I don't care for that approach, whether it is with a class of one or a class of 6. While independent learning is always to be encouraged, an entirely hands-off approach is really not instruction. I liked having a hands-on instructor in the pool to offer advice on skills techniques that he had found functional and dysfunctional while working with students. I fully agree with the 'actual vs simulated' student comments. If the LDS where you are planning to take the course is not doing enough OW / AOW / Rescue training to offer you internships with actual students, you might consider looking elsewhere. Simulation is valuable but not a substitute for real-life student experiences.
Your role and status as a DMC should be known up front. I expected to be a bit of a scut puppy when helping with classes, and didn't mind doing it at all. Slepping tanks and gear, handling shore duties that allow instructors to focus on instruction - all that is a reasonable expectation, and not a problem (when agreed up front). In one AOW class, I took my drysuit inflator hose off my reg and gave it to the instructor, who had forgotten hers, and made do with a bit of squeeze so we could complete the Deep dive. In one recent case, I had a DMC say that he wasn't 'free labor', he was 'here to learn' when I asked him to position tanks prior to a class. That was simply a 'failure to communicate' which was quickly corrected. But, there can be an excess of use of DMCs as free labor, to the point of extending DM training to get more out of the candidate, and you should ask about how long the program will last. Finally, I really enjoyed being one of a group of learners. It was fun, and we helped each other in the pool, helped with classes. Being one of several is not essential, and one-on-one mentoring is fine. I just found it to be enjoyable.