No one wants a bad DJ for their club, bar or event (especially when every Tom, Dick & Nancy is a DJ these days), so how do you know you've got a good one? First of all there's good and bad in every entertainment industry and the DJ industry has a very wide range of DJ types & DJ skill levels and basically if you're a bad DJ you won't last too long, especially in this day and age of social media.
To define good and bad DJs varies on many factors and one of the main factors would be what music and event they're hired for. As stated earlier there are different DJ types & levels of skills, so making sure you've got the right DJ for the right job is important.
The most basic but important factor that separates the good from the bad is how well the DJ can read a crowd, know what to play and when to play it (this in DJ terms is called vibe-ing, throwing down, zone-ing). No matter what the occasion if the DJ doesn't have people dancing or the place feeling the vibe, it's not happening and you have yourself a bad DJ :(
In my opinion (from a perspective of an underground DJ) and what i would call a good DJ is a DJ that has passion for music and a passion for what they are doing (DJ'ing). I myself have high expectations of a good DJ as i do myself (DJ Lambam). A good DJ has a good ear for good music (doesn't just play the same music or same versions as most mainstream DJs), can beat-mix 99% on-point, scratch but not essential and can make any occasion vibe with their specialised groove energy.
So with all the information online (social media), recommendations and hearing specific DJs names around the traps it's more than likely that bad DJs are weaved out and you're bound to get a good DJ.
My tip, don't forget to give the other not so popular DJs a chance cause sometimes they're just as good, but the overall rule in the DJ industry is...YOU GET WHAT YOU PAID FOR!