Things to consider when checking out a prospective optometrist include the level and range of their experience, their education, their board certifications, their reputation, and (if you have any special conditions to consider) the amount of experience they have treating your specific needs.
Like any service provider, your optometrist ought to be in good standing with any local trade organizations for their profession, and ideally the local chamber of commerce too (though they might not be a member, due to the expense). It's easy to check with a few phone calls.