They will either have years of experience or professional qualifications or both.
 

They should have testimonials and reviews from previous clients.
 

If the books they've edited have been published, they should provide links so that you can read the reviews.
 

They should offer a sample edit of ~1000 words before starting to edit your manuscript. This can be a free or paid service, but it is crucial for both the writer and the editor—if the editor doesn't feel they can edit your manuscript for whatever reason, or if you don't like the editor's style, you can amicably part ways before any contracts or payments complicate your relationship.
 

You should be given a contract to review and sign. This is, again, just as much for you as it is for the editor. It will outline the exact scope of work you are expecting the editor to do, set a timeline for editing and payment, ensure confidentiality of your manuscript, and lay out terms and conditions of the work.
 

Watch out for editorial rates. The Editorial Freelancer's Association provides a rate chart from surveys of its members, as does the Chartered Institute of Editing and Proofreading, and these are widely considered either standard or the low end thereof. If an editor is charging far below these rates, they are either not a professional and are using AI, or they are, whether intentionally or not, undermining their editing colleagues.

You shouldn't be overcharged, but remember that quality editing is done by professionals who deserve to be paid a living wage for their expertise and service.