A good disc jockey should take pride in their work, which is why you should endeavor to separate the backyard DJs from the true professionals. One way to tell is by the price they offer. If they say they can do your show for $50-100, they're the backyard variety - or worse, they're lying.
Be absolutely certain that your prospective disc jockey owns music that's appropriate for your event, and is willing to play only that music. Certain songs shouldn't be played for little kids, or at weddings. Sit down and discuss this issue with them before they every play a note.