You should expect a response to a phone call or email within 24 hours. The only exception may be on weekends when most DJs are at events. You should also have a direct line of communication with your DJ, i.e. their phone number, email, etc.
Your DJ should tailor the music for your event to your tastes. Your DJ should also have the musical knowledge, song selection, and willingness to play any genre of music you want for your event.
Your DJ should be able to provide pictures of their setup and equipment if asked. The booking and payment processes and terms should be clearly stated and explained up front.
Your DJ should be polite and courteous to you, your guests and other vendors. They should also be able to work seamlessly with with all other vendors, i.e. planner, caterers, photography, etc. Your DJ should be professionally dressed and dressed appropriately for the event. The equipment area should be clean and well-maintained. Your DJ should arrive early enough to have all equipment set up outside the presence of guests.
Your DJ should be flexible in scheduling meetings and phone calls. Your DJ should also be adaptable to changes, even last-minute changes, in the timeline, schedule, etc.