I shoot A LOT of weddings, and I have worked with A LOT of DJ's. Some of the DJ's I've worked with have been amazing, others I have barely noticed, or even disliked. Here's my advice on some things to look for.
1. REVIEWS, REVIEWS, REVIEWS
This should be a big thing for every vendor you research. Can they provide reviews? What about an individual bride that can talk to you about their experience? Make sure reviews are there, and make sure they're real by following up or asking to speak to a bride. You can also ask for recommendations from other brides by posting in bride-to-bride groups on social media.
2. What is their equipment like?
It's important to think about what you want your dance floor to look like. If you don't want weird colored lights in your photos, let them know. If you do, make that a must. Think about what your venue looks like and how their equipment would impact the venue.
3. Are they insured?
You do not want an accident with lights or sound equipment to injure a guest or a part of the venue. Make sure that every vendor you hire carries individual business insurance, typically 1 million dollars MINIMUM.
4. What type of music?
What type of music do you want played? Do you want them to play a list that you'll be giving them, or play music they choose? It's important to make sure you have this discussion. You don't want to go into your wedding thinking it's going to be Disney themed music and then find out your DJ doesn't have any Disney music.
5. What type of DJ are they?
This is probably the biggest thing that you will want to find out from reviews and testimonials. In my opinion, a DJ that is good will not just play music, but keep the party moving. They need to know when it is appropriate to suggest cutting the cake, first dances, and other fun stuff. You are going to be so caught up in the moment that you'll forget there's things to do. A good DJ can tell when the crowd starts to get bored, which may sound harsh, but your guests and vendors will thank you for choosing someone who can read the crowd.