Same as above. Ask a few sellers (there are plenty to choose from) for a quick audit and ideas. Then see what they send you and how much effort they put into it.
If they come right away with personalized and relevant suggestions for your business, they're the deal. If they only send you generic advice, move on. A good SEO expert will genuinely care about what you do and adapt their services to your site, rather than apply a one-size-fits-all solution.