Hire someone who has been in business for a while and is confident in their work,
Ask your neighbors who the best gardener they know is, and if that's not an option go to local chat sites like Yelp or Angies List and find reviews,
Check out past clients by visiting their website or portfolio on home services app True Local provider finder, ask friends on social media for recommendations of people they might have dealt with before.
Lastly always look to see what their personable attitude is like too because who knows how well the gardener will take care of you when things get tough!