This is a really good question as we are dealing with the human condition. You may have hired the best service out there but even the best can have a bad day. I’m not saying this happens a lot but you need to check your options. Make sure the service has good equipment. It does not have to be brand new but it needs to be solid & well maintained. You can look over their testimonials & references. You can follow-up with key questions about how they conduct business & you can look on-line for reviews. There are plenty of places you can look to find the information needed.

There are a few more things to think about. The new guy on the block starting out is not always a bad choice provided they are willing to listen & do what you need. Maybe all you need is a song list played or your event does not require a super mix dj. That is when I say go after the new dj just getting into the market. If they are smart they will make the best of this for the reference. I have been involved in all kinds of events & to be honest not all of these events needed a dj as much as they needed a jukebox or an ipod. I like to work when my skills can be seen & appreciated but not to be put in a corner & be told to just push play. The event planner needs to think about what they really need.
So to answer the question, You should always look at the service in question & what you are getting. Is their equipment well maintained? How long have they been in business? What kind of service are they? Check their history & online references. If you know your needs & budget & follow these steps, You will get what you pay for.