You want to alert them you have something that is on a particular format, they might want a copy ahead of time in order to better accomodate you. These questions are basically what determine if you can get the night. You should ask how long they can plan, and make sure you undestand how many hours your going to get. This isn't a bad question to ask, most DJs should be able to refer you to someone else who has hired them. You should also be leary of paying a lot for a relatively new DJ who hasn't established their services.
Some DJs can even provide a cassette tape of their services upon request.
A professional DJ will have equipment you have never heard of most likely. That is because their equipment is commercial, designed for rugged handling. Try and note the manufacturer and do a quick search at Alta Vista for the name and see what kind of page it shows up on. These are based on visitor comments, not my personal experieces. He really added a lot of energy to our party and everyone had a blast.