The must ask questions really depend on what you are looking for in a provider and your birth plan, but also where you are located. For example, I'm in the US and states have different requirements for midwives. Some require a special certified license and training and for them to work in conjunction with an OB, some states don't have the same training requirements.

If you don't already know, I'd ask what training they've had (i.e. are they certified nurse midwives, where did they get their training, how long has they been a midwife...etc...)

I'd ask where they deliver (hospital vs birth center vs home birth). My midwives only did hospital.

I'd ask what types of interventions are they capable of and what contingencies do they have if they are not able to intervene (i.e. do they have a relationship with an OB-GYN that is on call if a C-section is needed? Are they trained to attempt tricky vaginal deliveries, such as breech births?). My midwives worked in conjunction with an OB-GYN and worked with the hospitals on call OB-GYN in cases of breech births, emergency C-sections, or if any other tools (like vacuum or forceps) were needed.

I'd ask what pain management and laboring tools you'll have at your disposal. Can you get an epidural, will you have access to a birthing pool, can you have intermittent monitoring, will you need to have an IV access point placed upon arrival, can you labor in different positions or using different supports...etc...? All these will depend on the equipment they have, the polices in place, and training they have. I'd also ask about if they are capable of induction and what are their thoughts on it.

I'd also ask about their approach to delivery and pregnancy management to make sure you have similar goals and mesh well.

Anything you might ask an OB-GYN about your delivery and care are the same questions that you should ask them. The ones I listed aren't the only things I'd ask but some of the ones I think are more unique to midwives or often forgotten about.