There are a lot of questions you can ask a prospective search engine optimization consultant to gain perspective about their knowledge, skill and experience.
To me, one of the most important of these questions is: What are your most important SEO techniques?
Worst Answer: It's a secret.
If your prospective SEO consultant won't tell you what they plan to do on your behalf, politely move along. In the best case, secretive SEOs will "do stuff" that will have little or no impact on your site's visibility within search results. In the worst case, they'll get your site de-indexed (read kicked-out of search engines).
Sure, if you're lucky you can find one of the rare true Black Hat wizards that are able to consistently stay one-step ahead of search engines. More likely, you're working with someone who doesn't really know what they're doing.
And so, your likelihood of success is much like tossing dice on a craps table.
Better Answer: Links, shares and citations.
If your prospective SEO is talking about building links, shares and citations, they're on the right track. But you should quickly follow-up with questions about where and how they plan to build these links, shares and citations. You see, not all off-page search signals are equal. For example, links from relevant authoritative websites are much more valuable than those from sites that aren't topically related and don't have much web authority.
In fact, many SEOs that have a basic understanding of search engines will build huge quantities of largely worthless links. They'll also employ this quantity over quality strategy in generating social shares and business information citations.
Even Better Answer: Content development and marketing.
If your prospective SEO's answer doesn't somehow involve content development, I would suggest that they're either not familiar with the most recent changes to search, or they're planning on pursuing tactics that violate search engine quality guidelines (which is another question worth asking).
That's not to say that good content alone will propel your pages to the top of search engine results. Effectively marketing your content so that it attracts views, links, shares and other online publicity is almost as equally important.
But focusing on developing strong web content and getting it in front of people who are actually interested in it and have the means to further publicize it, should be core to any search engine optimization campaign.
(Photo by: https://www.flickr.com/photos/bilal-kamoon/6835060992/)