Let me establish the first thing - everyone on Codeable knows their craft. They wouldn't be here if they didn't. The screening process is long and difficult. Only the top 2% of WordPress developers worldwide make it through. Now when that is clear, we should explore some other criteria.

Do you need rather a backend developer or a frontend developer?

There is a difference between these 2. A front-end WordPress developer handles everything, that is visible/visual on the site. He designs and codes the layout. He makes sure your website is responsive, he adds animations and effects.
A backend WordPress developer's work is not that much visual. He codes the functionality of the website. He makes sure everything works as it should. He integrates different plugins and systems.

They often overlap

I'm a good example. Even though I'm mostly a backend developer, I can do front-end work. I would be strong in integrating 3rd party API with your WooCommerce store. But if you are my client and you do need to change a color of a button, I'll help you as well.

Codeable status

The backend and frontend WordPress freelancers are very well balanced on Codeable. I would say, the ratio is 60/40.

Is the WordPress developer a good communicator?

This is important. If you can't explain your ideas to the developer, he won't make them happen. Many people think that if they just hire a great coder, the rest will work itself out. But that's not true. Coding is only a part of the job. The other part is understanding what the client needs.
Thus excellent freelance WordPress developer also needs to be an excellent communicator. The better you both are in communication, the less overhead there is. This leads to faster communication and faster delivery of the project.

Codeable status

On Codeable, there is this rigorous vetting process. One of the acceptance criteria is how good of a communicator is the WordPress developer. The guys behind Codeable know that this is super important, to succeed.

Are your timezones compatible?

The client and the WordPress developer don't need to be in the same country or even the same continent. But there should be at least 3-5 hours of common working time.
If you are in Europe and the WordPress developer is in Australia, that's not going to work out. Communication will be very difficult. You will have to schedule everything in advance and there will be a lot of waiting involved. The project will take longer and it will be more expensive.
The best-case scenario is when the client and developer are in the same timezone. This way you can just pick up the phone and call each other whenever you need to.

Codeable status

The vast majority of the developers are from Europe and North/South America. I would also guess, that the vast majority of the clients are from these continents as well. From my experience, cooperation between Europe and America is super smooth.

How do you feel about the guy?

You will build a long relationship with the WordPress developer. It's mutually beneficial. So it's important that you feel comfortable with each other. You should feel like this is a person you can trust. You should like the person's style. Omit, trust your gut feelings.

Codeable status

There are currently 500+ WordPress freelancers. I've met 100+ of them in person. And every one of them was a great and honorable person. So whoever you'll pick, there is a good chance it will be great.