1) Browse the Galleries: Not all photographers are created equal!
This is super important , as all photographers each have their own style or feel (aesthetic) to their final images. Some work with equipment in a studio, and others work on location utilizing the natural light only. Make sure prior to booking, you browse their website and take a peek at their photo galleries to make sure that their style matches your expectations on what you'd like your final images to look like.
Do you like how they edit? Do you like their poses? Look at the background of the images. Are there a lot of distracting elements? Do you feel like you got to know the people from that session just by looking through their gallery?
I know all about a budget. Cost before aesthetics for some, could be the deciding factor when choosing a photographer. With that in mind, it is important to figure out exactly what is included in the total cost of the session. Sometimes a photographer may look inexpensive, however, once you factor in prints and any additional session/package fees etc. - they could end up being the most expensive. If something is unclear ask for clarification before you choose
3) Referrals from friends
Word of mouth is an excellent go-to source when searching for a photographer since the person who referred you, will have already gone through the entire process. You can ask them questions like : Were they happy with the way the photographer interacted with their family? Did they get their images when promised? Would they use their photographer again? Someone who has gone through the entire process with a photographer will be able to gauge the experience from start to finish.