As a SEO professional whose experience comes back to the year 2000 in optimizing Websites, I get this question in many different varieties. Sometimes it's presented within the context of a presentation I'm giving. Other times it's presented within stealth 'Pick your brain for free "conversation I may have the displeasure of participating in (Smile).
The first thing I'd like to point out is that SEO currently is probably the least professionally skilled that someone can obfuscate or dissemble about. For those who prefer more direct language, it's the hardest job today that you can not tell the truth about what you've done. Why do I say that? Search Engine Optimization is about getting your customers Websites ranked in the Search Engines. Anyone in the world with an internet connection and a computer can see if you really know what you're doing; they just have to type in the appropriate search terms.
That being said, it brings us to the first major point; If someone claims to know SEO or be a SEO consultant, pretend you're from Missouri, the "Show Me" state, turn your computer around and ask them to show you their rankings. Verify that the sites they tell you they've optimized in fact sites they've worked on, and actually look at their rankings. A true SEO will jump at the opportunity to show you this, because they will have achieved excellent rankings for their customers. Someone who has read a book or a couple of articles and now purports to be an SEO expert will wince at this request for proof. Most likely because they will have none to show.
The second major point in verifying the status of a true SEO, if they pass the test above, is to then look at the quality of their rankings. Relatively speaking, it's significantly easier to rank a Website for 5 and 6 word terms and key phrases than it is to rank a site for one, two, or three word terms and key phrases. Now do not discount the value of these 4-8 word terms and key phrases, because these are collectively known as "The long-tail of search" key terms. The "Long Tail" gets its name and significance due to the fact that these terms usually indicate someone who has done their initial research and is ready to make a purchase, take and action, etc. However, a true SEO should be able to show your rankings throughout a range of the number of keywords that describe your site (s).
The third major point to look for is if a SEO firm / Consultant can pass the test for both of the points above, then break down the number of documents in the index that their optimizations are competitive against.
Again, relatively speaking, it's easier to rank two or three word terms or phrases that have 3 million or fewer documents in the search engine's index rivaling against them then it is if there are 50 million, 100 million or 200 million documents in the index competitiveness against them. I've encountered profitable, mid-size search engine marketing firms that have posted case studies on their websites about how they've ranked a major client number one in Google for a particular term that had all of 2.3 million documents competing against it. I almost could not stop laughing. I've ranked major companies number one, two, three, four, five, and all positions on the first page with over 100 million – 200 million documents in the index competitiveness over them. I've actually had their competitors copy my optimizations word for word (I can show you proof of this if you'd like. ). In the process of copying my optimizations, these same competitors have garnered number two, three, four and sometimes even number one rankings themselves.
So in summary, there are three major points to look forward if you're trying to find a real SEO firm. They are;
1. Have them show you their rankings
2. Make sure their rankings include one, two, and three keyword phrases
3. Make sure those rankings are achieved in an environment where there are many documents in the index competitiveness over them
If you stick to these guidelines in evaluating your next SEO firm and / or consultant, you should have no problems in selecting the proper firm to fit your business SEO needs.