In the UKBF SEO section you often see posts from people who are bemoaning SEO's, suggesting they've been ripped off and whining about how the language of SEO is confusing and misleading.
In essence you can't start looking at SEO and immediately have a grasp of the many aspects of it. For sure a good SEO can explain the concepts in plain English - we certainly try to - but nonetheless some aspects of the subject are difficult to understand.
So when you are looking for an SEO supplier engage with them, ask them to explain in simple language what they do and how they can help your business. There are plenty of snake oil salesmen involved in SEO - just like most areas of business - but as with the other areas of business asking questions and creating a relationship should give you a feel for the calibre of the company you are thinking of engaging.