You sound qualified.
However, the best proof is in the results themselves. If you can point a prospective client to a specfic keyword and they can find you (or client of yours) from their computer in top 10, or better, SERPS, that will usually sell them.
*Specific keywords should have 1m, plus competitors to significant.
SEO, of course, is just 1/3rd or less of effective eMarketing. Who cares if you get all the traffic and no one calls, or completes a form, or buys a product?
Be ethical, work all phases of eMarketing, and be concerned about your clients ROI. A long term relationship will result and that's better than 100 angry customers.
Best of luck.
Michael.