Search online and read reviews based on customer feedback. Give them a call and see what they offer based on the packages. They should have sample music that can be provided by email. There was something brought to my attention when i had booked my dj, that is being certified and proof of certificate.
I google searched a bunch while asking for recommendations on a Facebook post as well (I'm part of a city Facebook group). Once I had a handful of names, I gave them a call for prices and from there had two in mind. We picked based on how personable they were and asked to listen to a short mix so we can get a better feeling for them.
Check reviews. Facebook wedding groups are a great place to ask for recommendations!
DJ’s should have mixes they make regularly that they can send you. If not, let them know what genres you’re interested in having and get them to make you a small mix. I had a couple DJ’s do that for me !
Ask your venue about dj recommendations. Referrals from friends. I read a lot of reviews online as well. But it helped that I had a couple friends recommend the same person. You can request a meeting/consult with them too.
We chose our DJ based on recommendations from our photographer. We figured that they have been to a lot of weddings so they must see a lot. They offered us a list of recommended vendors for hair, decor, makeup, cake, DJ, etc. It was very helpful, so if you have your photographer booked I would try to ask them to see if they have advice. After we had the list we went through online reviews and chose the DJ based on the reviews on the Facebook page for his business.