I am a doula. Here are some things that I think are important to know and discuss -- Certification is not necessarily in indication of quality -- there are many very very gifted and experienced doulas out there who are not certified for various reasons... training is much more important than actually having gone and filled out the paperwork. Definitely check references, but remember that if someone isn't very experienced, it doesn't mean that she isn't very good. Having a connection with your doula is extremely important. Make sure you feel comfortable with her. Do you get a good feeling from her? Or does she remind you of your Aunt Zelda who you always disliked? (you know what I mean.) Is this someone who you would feel comfortable seeing you at your worst as well as your best? What are you looking for? Do you need someone who is warm and fuzzy, or do you need someone who is more of a take charge kind of personality? Do you want a younger woman who will feel like a peer or an older woman who will feel more like a mother? Does she seem to come with her own agenda or does she seem to be focused on finding out what you need? And remember, what you think you need now might not be what you actually need during labor -- is she flexible? Ask her how she will support your partner in supporting you. Does she have backup? Is is possible for you to meet her backup? In the case of an extremely long labor, will her backup take over for her? (I work with a partner, and at extremely long labors, we might spell each other for a few hours because in total honesty, after about 24-36 hours without sleep, neither one of us is particularly good support anymore, and it is in the mother's best interest to have a fresh doula at that point). What is included in her services? When does she expect payment? How long does it usually take her to get to a birth? Does she do at home support if you want to labor at home for a while before going to the hospital. (my assumption is that you are planning a hospital birth... if you are planning an out of hospital birth, ask your midwife for recommendations for doulas she has worked with before.) Are you planning on using any specific method of birthing (hypnobirthing, bradley, lamaze, birthing from within, etc.) and is she familiar with it?
I'll probably think of a dozen more questions over the course of the day, but these are just what came to mind immediately.