Cutting right through the chase is a good bet for putting a prospective SEO agency on it's heels. Odds are, if you're talking to an account executive, they will have no idea how to answer you. And if they do, they've probably received a little training from the in-house SEO, if he/she even exists (you have to consider the possibility that the potential SEO agency is outsourcing their work overseas for pennies on the dollar). You're looking for pretty specific answers with this question, and the sales person you're speaking with should be able to answer very concretely without confusing you. Linkbuilding is the fundamental element of SEO, which means, it takes great skill to do it properly. If the candidate your screening mentions they comb through competitor backlink profiles, seek out blogs relevant to your niche for links, investigate quality forums or directories, and mention developing content, sponsorships/partnerships, or blogging - they're on the right track. If they fall back to "directory submission" or come up with some confusing monologue about blogging which doesn't really make any sense....it's time to promptly move to question two...