First and foremost, you can judge the quality of a WordPress Developer based on their website. Things like how fast the site is loading, how good the content is, how well the site is performing in SEO is a tell tell sign of their expertise. Also have a look at their work, testimonials. If you are still not sure, ask them to provide you with a reference and talk to their client about their services. If still a concern, make sure you got the deliverables in contract or any form of written communication.