I can count on one hand out of the many weddings that I have done when the DJ was setup on time. They have a lot of equipment and some of them did set up early but didn't do mike checks until it was time for the ceremony to start. They set up their equipment and left to get dressed or do another event. When the ceremony began the officiant wasn't able to immediately begin because the sound was not ready.
You should put in your contract what time you want the sound to be ready to start and add a deduction clause in the contract where money will be deducted if that is not accomplished.
Also ask for references from the DJ before choosing one. Another suggestion is to choose a DJ that has a site that permits reviews. That way you can see what live reviews have been added to the site and they will be accountable for their work. Try to find one that is listed on a big site like The Knot or Wedding Wire. They allow reviews and even the free listings have them. Good luck.