They say that doctors make the worst patients, right? I wonder if that translates to photographers. Here are the things that I look for and ask about when I'm hiring a photographer to take a portrait session of my own family.
First and foremost, make sure you like the style of photos from that photographer. Every photographer has their own distinct look in how they compose photos, how they pose families, how they process photos, the photos they select for delivery, etc.... If you find yourself directing the photographer with questions like "Can you add that hazy look?", then you should probably look elsewhere and find a photographer that naturally has the look you want. If you try to change a photographer, you'll move them out of their comfort zone, and you won't be happy with the results.
Look at a Full Session
If you are only looking at a photographers portfolio, then you might not be getting the full picture. A photographer is only going to put their very best work in their portfolio, and as the saying goes, sometimes even a blind squirrel finds a nut. Bad and mediocre photographers might appear to have a decent portfolio. Instead, ask to see a few complete family sessions. You'll get a much better idea of the quality of work that photographer delivers.
If having digital copies of the photos is important to you, then make sure the photographer offers them. Ask if they are included in the up-front price, or there is an additional fee to purchase them. Be sure that the digital photos you receive are not watermarked. It may also be important to ask what resolution (size) the photos will be delivered at. Some photographers offer lower-resolution versions initially with an option to purchase the higher-resolution versions.
By default, the person that takes the photo owns the copyright to that photo. Be sure that the photographer is transferring the appropriate rights to you. This is usually specified in the contract. Here are some typical rights you might want: Share on social media, share on a website, print, share with a commercial publication like a magazine.
Storage and Backups
You might want to ask about the photographers backup and storage strategy. Specifically, you might want to ask how long the backup photos, and the request process in the event that you need them. This might be especially important if you don't have access to download the high-resolution version of the photos and you need to go through the photographer for prints.
You should ask a photographer how long after a session they deliver their photos. If you need the photos in time for a specific event (Christmas cards, show at a party, etc...) you might want to set a specific delivery date. Also, you might want to ask how many photos they typically deliver. They might not be able to give you an exact number, but it'll at least give you an idea if it's 5 photos or 100 photos.
You'll probably want to make sure your family will mesh with the personality of the photographer. To get an idea of their personality before making your decision, you can read online reviews, speak to friends that have used that photographer, speak with the photographer on the phone, and even ask for a face-to-face meet.
You might want to ask if the photographer has liability insurance. You want to make sure he's covered in the event that something unexpected happens. Perhaps you accidentally break something at the location/venue. Or maybe the photographer trips and breaks his equipment or hurts himself. If the photographer is insured, there is less chance that you'll be left holding the bag.
Lastly, you'll want to make sure the photographer has a standard contract agreement. The contract isn't meant to only cover the photographer, it also specifies your rights as a client. If there are any issues with the session, the contract should take any ambiguity out of the situation. The contract should specify the session details like the time, date, location, length of the session, etc... If you took my advice and asked about all of the above items, you'll want to make sure any specifics you've agreed on are specified in the contract. For example, a contract might specify that you'll get your photos in 2 weeks from the shoot, or that you have rights to share them without a watermark on social media, etc...