Go see the DJ's performance. "The proof is in the pudding."
Get several references. (Finding out if a DJ is reputable is on of the most important things you can do.)
Obtain a contract. It will protect you and the DJ. It proves that the DJ is serious about his work.
Find out how many years of experience he has.
Ask if he offers any of the extra services that you might want.
While speaking to him try to see if his personality matches what you're looking for.
See if he has back up equipment in case of technical problems.
What A Professional Mobile Disc Jockey Can Offer:
MCing abilities and quality performance
Good reputation and years of experience
Spends money on improving his business
Has extensive music library and keeps up on new music lists
Has judgment to play appropriate music at appropriate times and volume levels
offers extras: radio edit music, specialty music, Karaoke, dance lights, intelligent lights, up lighting, fog/haze, video shows, interactive games, props, novelty handouts, game shows, dancers, multiple DJ's, rental equipment, photography, videography, lead/teach dances and the list goes on