Whether you are working with a freelancer or a big agency, they will have a portfolio for you to view. This allows you to see other, similar projects they have worked on and decide if they have the skills you need.
There are a number of web design certifications and they may be happy to share any they have attained, but there isn’t one standardized industry body.
If you are worried about a designer’s credentials you can also check their feedback from other clients, which you can view on their Bark profile.
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