Weddings may be notorious pick-up spots, but your DJ shouldn't be hitting on anyone before, during or even after the event. This is business, and making lewd comments from the stage or chatting up the ladies during a song is not what you're paying him to do.
So, how do you know if you're hiring a professional disc jockey or a Casanova with a mic? Pay attention to how he conducts himself during your interview. Was he ogling your maid of honor? If you went to a restaurant, did he hit on the waitress? Did he seem to pay more attention to your chest than your song requests? If he seemed to have sex on the brain during your meeting, it's only fair to assume he's going to be more concerned with mixing it up with your lovely bridesmaids than playing the right mix of songs at your reception. Just go ahead and let him watch you walk out the door, because it's the last time he'll be seeing you.