This is going to be a disagreed with opinion, but I don’t care. This is how we do it.
We are an SEO agency. We get you to the top of the search engines for good keywords, etc. We demonstrate our worth by getting you to the top of the search engines. To do this, we use a SERP tracker (I like serpfox but there are heaps). A SERP tracker definitively proves if you are doing what they hired you to do or not.
If we get you to the top of the search engines for the right keywords, and you do not make more sales, then you have either a product concern or a conversion concern. We do not do either of those. We are not a conversion agency. We are not a product development agency.
Too many SEO agencies try to be everything. They want to do SEO, and they want to do conversion, and they want to do website design, and they want to do content, and they want to do paid search. And usually, none of them are successful, or they just outsource it. A lot of our clients are big SEO agencies who don’t even actually do SEO themselves, but outsource it to us. wtf. Anyway, you prove your worth as an SEO by getting your client to the top of the search engines for the best keywords.
The only way you are at fault is it you select the wrong keywords, or if you get no results.