There are some general attributes and qualities you should look for when you are hiring a DJ, wether it be a personality trait or a business quality - you need to keep these things in mind.
Underline. Bold. !!!. Experience is key. Playing music for a wedding is different than any other DJ set you would play at the club or at a house party. Understanding the flow of weddings and knowing who to please and when is a refined skill you learn from years of experience.
An Extensive Song Library
If your DJ only has a couple hundred songs on their laptop, I can guarantee they don't have the song you want to hear. And even in the modern age of smartphones, the internet goes down, and downloading and streaming isn't always easy or even an option. Having an extensive library makes it so I have 99% of requests at my fingertips and I can throw them onto the dance floor in seconds!
Ability To Improvise
Sometimes the music that was planned, or the playlist the bride and groom thought would work isn't working and you might need to be ready to throw down an hour of hip-hop even though you prepared 3 hours of country two-stepping. This can also be a throwback to experience, but being able to change plans at a moments notice while staying professional the whole time is a must-have skill for any DJ.
A lot of people like music. But your DJ needs to KNOW music. At the very least they need to know the words: chorus, bridge, verse, key, and tempo (BPM). Having someone with real music knowledge not only benefits any mixing the DJ will do (Listening to music mixed out of key all night can give you a headache) but it also comes in handy during the not-so-rare guitar/vocal performance or karaoke dedication.
If you see any red flashing lights... your DJ doesn't know how to use his gear.
To the benefit of your event you need a DJ who understands gain stages, feedback loops, microphones, various audio and video input, outputs, and converters.
Turning down requests that would ruin the fun or that are on the bride and grooms "Do Not Play" list get's more difficult the longer the bar has been open. An easy going DJ who can dodge bad requests and suggest better options will keep the party going smoothly.
Did your DJ download your first dance ahead of time are are they hoping the venue has wifi? It takes a good amount of preparation time to gather your requested music, review your itinerary, organize special requests, and plan out a successful wedding playlist. An organized DJ is a successful one - you can tell a lot about a DJ by the playlists they have built. Any professional DJ has a playlist for every era, genre, mood, and theme.