This is a great question. Frankly, there is no way you can blindly trust any company or freelancer. However, in my 15 years of career as SEO consultant for multiple agencies, I can tell you that SEO is no longer a one man show. This is a team effort where everyone from SEO Analyst, Industry content writer, link outreach specialist and technical SEO expert will work together to deliver outstanding organic traffic for your website. However, if you are looking for just SEO consultancy, you can hire an independent SEO consultant or an Agency or a freelancer, but only after reviewing his reputation / results which they have delivered in the past.