A major point to make to you is, Are you comfortable with them? Will they provide the service you are in search of? Plain and simple, you have to go with your gut on this one. This, by far and away, will be the single most important vendor you hire. They have to provide the flow of the evening without being the spotlight or center of attention. They have to work with all the other reception vendors to keep the flow of the evening going, on time, get people dancing for your video, provide the right mix of music for your day and keep your guests, ranging from 5-95, happy with each and every tune they spin. 
Everything could be perfect! Then, your DJ/MC, doesn't show up, cannot get people motivated to celebrate with you, and does not have the right first dance song. It has happened. 
Quick, what was the last wedding meal you had? What colors were the flowers? How many layers did the cake have to it? Unless you are reading this the day after you just attended your best friend's wedding, you will be hard pressed to remember those items. Now, did you have fun? Did everyone enjoy himself or herself? Easily remembered, and my point. Everything could be a disaster, but if the entertainment does their job, then everything was perfect in the bride's eyes. 
Here are a few questions to ask: 
1. Professional equipment?
-Home and store bought equipment is not designed for everyday use. 
2. References?
-Everyone should have references 
3. Can I see you perform at a wedding?
-Personally, I wouldn't invite anyone to your wedding, however, some companies do allow it, so ask it.(Not a deal breaker!) 
4. Do you have a video?
-Again, some do, some don't. Remember, they can be edited! 
5. Can I make requests?
-Should not be a problem, however, remember, you are hiring a professional to perform for you, it is his or her responsibility to make everyone happy. So 4 or 5 extra requests is a good number. 
6. Do you have the most recent music?
-Most DJ's belong to record pools. They keep their library updated weekly. 
7. How many weddings did you do last year as referrals?
-Great question to ask. If they are being referred by other brides, and frequently, they must be pretty darn good! 
8. Do you belong to a trade association?
-Some very serious DJ's belong to a national association. (Not a deal breaker, so if they don't, don't sweat it)! 
9. Are you insured?
-Professional Disc Jockeys show their true business side on this question. A fully insured DJ service will have the comprehensive to cover their equipment AS WELL AS liability to cover themselves, the guests and the Bride and Groom. Let them take care of this. Little piece of mind for you if someone accidentally falls, puts their hand through a door, or those other unexpected things that flair up. This is a no brainer hire a professional with insurance! 
10. Ask them the little things like you are interviewing a prospective employee, (Because you are) like how long have you been in business, how long have you been DJ-ing professionally, do you like it? Etc. (There are no right or wrongs here, just get a feeling for the personality) 
11. Have them tell you about their worst experience while performing at a wedding. No body is perfect, however, you will be able to see if they can critique themselves in order to make themselves better. 
12. Will I get the DJ that I interviewed, or someone else?
-This is a good question and they must provide the right answer. Multi operated DJ companies have many Weekend Warriors on staff. Will you get someone who will match the personality that you are looking for? DON'T TAKE THEIR WORD FOR IT! Ask them to interview this person. If they don't interview you, then don't consider them! This is a deal breaker! 
When interviewing a disc jockey for your reception, or band for that matter, treat it like a job interview. Whether you have experience interviewing or not, you are really hiring this professional for the most important part of your wedding reception, and once you hire them, and you are committed, it is very difficult to dismiss them, especially at the reception! So, make sure that you are comfortable with them, no, you don't have to work with them the rest of your life, but you may remember it for the rest of your life! (Good or Bad)
Disc Jockeys can range from a low of $500. to a high of $2500. depending on your town. What you want and where you want to be in that range depends on your budget and importance you put on your reception entertainment. Don't take this the wrong way, there are some very good $500 disc jockeys out there and there are some not so good $1500 services as well.
The most important thing I look for in a bride and groom when they are interviewing me is 'What they want' as far as interactivity, the flow of events, and what events they want and don't want. I also look for a feel from them if they are comfortable with my format and me or not. I certainly would not want to be someone they settled for or was just sold by, their event is just as important to me whether I am the entertainment service they choose, or another fine area service.
Ask them the difficult questions, like "What was the worst performance you had at a wedding reception?" "Can you give me any references that you may not be sure of?" (They may or may not have these, I randomly use all of my previous clients and really do not know what they say). "How much do you talk on the microphone?" (The answer should be just enough, this is not a radio show, and so the songs do not need any introduction or post messages) "What is your format?" (Let them explain what has worked for them in the past) "Can it be changed?" (If the answer is yes, you are on track, if they say no, or I don't like to, what will they do if everything doesn't go as planned? and most receptions can't follow a format to the exacting minute because of the fact of it is not rehearsed!) The best one toast a good DJ is "Can we pick all the music all night?" An experienced professional will tell you right up front, You can, however, they wouldn't suggest it because then if things aren't working out, your guests will leave and your DJ's hands are tied, Most will tell you to pick out 4or 5 must hear songs (other than the formal songs like First Dance, Bridal Dance, etc...) If they get a little uncomfortable with the question, then you have someone who knows that everything changes all night long, mood swings, tastes in music and interactivity. You have found a DJ that will probably do what ever it takes to ensure that your reception will be fun, energetic and enjoyable. Their business depends on it!
One thing that you will hear time and time again, Make Sure you talk to the actual DJ that is going to do your reception! (This is for the brides looking at a multi-operated DJ company, most have some very good DJ's on staff, however, make sure that you are comfortable with the DJ that is being assigned to you!)
Best Wishes for a Successful Reception!