I found this great article sharing these 10 questions to ask a DJ. Read the full article here.
1. Is this your full-time business? Are weddings/events your primary focus? What clubs, lounges, and corporate clients have you performed for in addition to weddings?
2. How do you customize the music experience for each couple? Can you help with song lists and providing suggestions?
3. How do you get the crowd pumped?
4. How do you handle song requests?
5. Can I hear some examples of mixing and blending different tracks?
6. What sound equipment do you utilize? Do you have back-up equipment?
7. Have you played at our chosen wedding venue before? If not, can you make a site visit beforehand?
8. Do we have our choice of DJs to select from, or is one automatically assigned to us depending on date availability? What happens in case of an emergency?
9. What other services do you offer (e.g. lighting, HD video screens)?
10. Why should I choose you as my wedding DJ?
Here's the rest of this great article: http://www.bridalguide.com/blogs/the-budget-guru/wedding-dj