Having been in the DJ business some time now, we feel it is important to inform people that the potential for being cheated exists in this business... and it comes from surprising places sometimes. (see summary near bottom - for quick reading)
One of our contacts were sponsored by a large radio station and were left hanging on the day of their event with nothing. The DJ just simply didn't show up - and didn't let them know ahead of time that he/she couldn't make it. Neither did they have enough time left to have another DJ set up for them, given the kind of setup they needed.
Other people end up worrying about DJs who they hire, double booking them and then absconding on delivering the services they've paid an up front deposit for. Another variation on this theme is the DJ who owns a fly-by-night operation, whose intent it is to obtain a deposit and disappear.
The list of possible horror stories could go on, and on - but we expect that people can only do so much. So, we suggest some simple measures here to get as many of these troubles under control as you can - short of simply saying "hire DJ-Raj".
Your DJ should be easy enough to get a hold of within a short period of time. Consider the numerous factors that could go wrong in a "mobile" DJ business - anywhere from equipment failures to transportation or health issues. In any of these cases - you would want to know as soon as possible - so you can chase down an alternative arrangement.
A good DJ will know that he is not able to make your show by about 1.5 hours ahead of your event, because that is the time required to setup the sound system for an event plus or minus half an hour. If your DJ doesn't have a cell phone in today's mobile business environment - you *may* be cutting yourself short. He/She may talk about the possibilities of spam advertising as an excuse not to advertise a phone number - but those are the costs of running a legitimate business.
DJs should advertise good means to get a hold of them. If they don't advertise a phone number at the very least, you may be setting yourself up to get work from a DJ that will stonewall you with silence if they don't deliver on their promises. The more the options for quick response communication, the better.
DJs should always have backup plans for almost everything they carry, especially if they bring computers to a show. Not all DJs have backup systems to ensure that your event will go flawlessly - and even some bigger outfits don't offer alternative systems. It is one thing to have the equipment and another to actually physically carry all of the backup options with you. In this case the adage "You will most likely pay for what you get", comes true. That isn't to say that our own business doesn't occasionally offer special deals - we do.
Given the number of events my company has performed, I've had to occasionally use my onsite backup equipment - because of the occasional hiccup on a computer or a piece of hardware/cable that failed. So, ask your DJ what kinds of things he has to help ensure that breakdowns are covered.
Summary: Expect to do a little homework when searching for a DJ. Take whatever he has to offer in the way of proof. Even if you are price shopping - you should possibly do a proof comparison too. Check to see that the DJ publicly advertises a quick means of communication - including a cell phone number. Ask your DJ, what backup options he brings to the table, if any.