1) Know what the photographer specializes in. You might think all photographers are created equal. This is not true. This is the most important thing to ask! A lot of photographers that are new to the industry, try to do it all. It is hard to perfect one area of your craft if you are trying to do it all. Newborns for example, are extremely hard to wrap and pose. It takes time too perfect these poses and to learn all the different ways to wrap. It also takes time to learn the tricks on getting them to sleep. I have never photographed a wedding and I don't want to. I know this would not be something I would be good at. There are a lot of things to consider when shooting a wedding. The lighting, which photographer (2+ photographers is a must for a wedding in my opinion) will be shooting what, location, timing of the day, what things to make sure you photograph throughout the day.
2) Ask for some recommendations. Hiring a professional photographer is an investment. You want a photographer that will make the experience enjoyable for everyone. Long gone are the days of awkward posing at JC Penny's, that the entire family hates!
3) Know the style of the photographer. Some photographers have a light an airy style while others may have a dark and moody style. Some are more creative and fine art while others are more casual. Check out their website to see if you like their style. What do you like about the photos? Are they simple and classic, match your clothes to the styles of what is shown. If you are unsure about the photographers style then you probably shouldn't hire them. Some photographers haven't found their style yet and are still trying to figure it out. Another common mistake in the industry is photographers who change their style to match the clients. You might see this as a benefit but it is not. If your style doesn't match theirs then the end result might not be what you were hoping for.
4) Look for a photographer with experience. There are many things to think about when you are taking photographs. Is everyone's hair in place, any bra straps showing, is baby looking at the camera, what is the best lighting, what is the best aperture/shutter speed/ISO for the scene, what is going on in the background, how to pose everyone to show connection and make you look your best, etc... These things take time to perfect! This is especially important for weddings, newborns or anything that is hard/impossible to redo.
5) What products/services does the photographer offer? Do you want wall portraits, albums, cards, digital images and do they offer that? Do you want services like hair and makeup? Access to maternity gowns and newborn outfits?
6) Ask to see a before/after image. You want a photographer that is capable of getting a good photograph without a lot of editing. Photoshop is a skill of its own and can really enhance a great photograph, but you want to make sure that they have the technical skills to produce a quality photograph, without a lot of editing. A skilled photographer will have no problem showing you a before and after.
7) Pay attention to questions they ask you. Do they care about what you want?