Finding the right DJ is much more than equipment and cost. A DJ can make or break an event, and is often the one thing determines whether or not people remember the party.
With the countless number of DJs and part-time DJs, its important to understand exactly what you want and what you'll get. That being said, the following tips (in no particular order) should help you make sure you've covered all your bases
1) Music IQ
A DJ should be able to read a crowd and understand what it will take them to get them moving and shaking on the dance floor. Any DJ can play top 40 music, however DJ's who understand that the Bride and Groom went to college together back in 2001 will get them and their friends moving a lot better by playing Boys 2 Men, Notorious Big and music that his/her audience can relate to. Make sure to ask your DJ for suggestions on songs to play for a first dance, cake cutting, grand entrance or any other function so you can gauge how they understand your event. Kool and the Gang's "Celebration" is a great song, but is done at almost every event! People love creativity and originality, just as much as they love a good flashback tune (Motownphilly is one of our favorites).
2) Professionalism
Make sure your DJ is responsive to your emails, has meetings with you before the event, and has a list of clients who can vouch for his/her services. The last thing you want is sloppy wiring in a hotel ballroom causing unwanted issues. DJ's should also dress as well as any other guest at the wedding. Suits at weddings, semi-formal at less fancy events. 
3) Prior Experience
Ask the DJ for a list of prior clients, or better yet, ask them if you can watch one of their events before yours. This way you can see them in action and make suggestions for your event.
4) Emcee Skills
A lot of people think the fact that they have a DJ playing music, people will get on the dance floor. This is far from the truth. It takes a good emcee to get the crowd excited and pumped up. They should be part DJ, part showman in getting the right people up there. Bonus points to those DJ's who are funny and can call out the situation (e.g "I know you guys have been enjoying the open bar that the hosts paid for, so get up there and dance with them!"). A good emcee can be the difference between photos of 2 people dancing or 200.
5) Mixing
Make sure the music flows smoothly. When the crowd is energized and the dance floor packed, the music should be fast paced (Disco, House, Some Hip Hop). When it's slower, slow song selection is good at getting the audience engaged.
6) Equipment
You're not required to understand the world of professional audio, but most DJs should have a laptop, professional looking speakers (i.e. nothing with massive amounts of scratches/duct tape covering them), and at least two microphones.
7) Contract
You're likely paying a nice chunk of change for your event, so look for a contract before hiring them for their services. Professional DJ's require a deposit, which indicates signs of experience. Make sure you understand what equipment and music your getting, as well as a firm understanding of what time the DJ will be there until and how much they charge in case your party runs late (normally an hourly rate). Secure a copy as well!
This is your night. Although not an exhaustive list, make sure you look for the things above when picking the company that will put the "Party" in "Party."
