What do you ask potential officiants?

Ask any questions you have concerns about your ceremony and how they work to make your vows and service work towards your style. They may have a package ready for you to look through what you like to put together of your choice. Ask about their price and payment plan.

One thing to have with an Officiant is the comfortability of the vibe you feel with them.

Ask them about their style, how they communicate, if they use particular language etc. Definitely check on any restrictions they may have with their service (some officiants are very hard core about keeping a particular style of decorum while they run things). You can also inquire about whether they have experience with the space you're using. Run some ideas with him and see if they will gel.

You should request video samples of past ceremonies they've done if they have any so you can take a look at to get a feel of the way they conduct their service.