Hiring an SEO agency. What should I look out for?

So I’ve finally decided to make the jump online to help my business flourish. I have some talented developers working on my website, and things are going smoothly so far. A lot of people have also recommended that I get SEO services too, so I’m planning to hire a (link removed by moderator) to help me. Does anybody know what specific things I should look for before I decide to hire one? Thanks!