To begin with, as we have had in our contracts since we began, SEO is not a science, but an art (or words to that effect). There are definitely known factors which influence ranking - the <Title> tag, <H1> tag (and through H6), bolding text, keywords in the first sentence and a bunch of others. That's on-page SEO and it's somewhat quantifiable. I say somewhat, because, with Google, the goal posts are continually moving. Content "above the fold" is a hot change this year. Many sites have rightly recognised this as prime advertising real estate and jammed it full of ads. Google says, "No, our surfer came to learn about <search phrase>, why should they be assaulted with ads?" So these sites have received a penalty.
Off-page SEO is as fluid as on-page. Link farms and link sites were hit hard this year. The traditional reign of back links has been diminished somewhat (though are still important) with more weight going onto social bookmarking.
Magician or artiste?
What is your expectation of your SEO person? Have they set your expectations properly? Do you see them as Merlin, ready to wave a magic wand (knowing all the "inside" secrets), and, "poof",
What color is your SEO's hat?