You know that guy who annoyingly sings -- or at the very least, lip-synchs -- every song that comes on the radio or plays in the clubs? Yep, that's your DJ, or at least it will be if you hire the wrong guy.
This trait's a little more difficult to spot than some of the others on this list, but there are signs. A good indicator that you're about to hire a wannabe wailer instead of a professional disc jockey is if he tries to sell you both his DJ services and his band, but that's not all you have to look out for. If you meet in a public place, and he's singing the background music, or he lists tryouts at a televised singing competition as related work experience, you probably want to tune him out and move on the next name on your list.