The first thing I would do is ask my friends, family, and co-workers. Get a few names to check out.
Then I would Google them. Look at their website. Is it professional? Do they have a large online presence or not? Look them up on Zillow, Realtor.com, Yelp, etc. Read their reviews. Look at their social media profiles. Do they post helpful information? Do they have a large following?
If they make it past all that, call them up. Do they answer the phone or return the call quickly? If you are looking for someone to sell your property, ask them to send you some of their listings. Do they look complete and full of details? Do they have professional photos? How about videos and a 3D walkthrough? Look at those addresses on Zillow. Is there a video walkthrough?
If you are looking for a buyer’s agent, you can also for information of what they've sold recently. Have they sold in the area you’re looking in?
Ask to meet with them. Meet with more than 1 agent. Do you like them? That’s pretty important. Did they come prepared with some professional materials for you to take?
Once you’ve made it to this point, you are probably safe to choose any of them. Trust your instincts. Pick a successful, prepared, professional, full time, experienced agent who you think you’ll like working with.
Best of luck with your sale and/or purchase!