You'd have to find out from them specifically what kinds of techniques they'd use.
Stay away from:
Cloaking (except in specific circumstances, but you probably aren't one)
Redirects of multiple domains
Those are the kinds of things that can get you in trouble with Google. Additionally, I'd stay away from anyone who emphasizes "meta tags;" it's a clue that their knowledgebase is out of date. Be cautious of anyone who guarantees rankings; usually that means rankings for useless keywords at meaningless minor search sites. Sometimes it means they're really talking about PPC, not "organic" search results. Those are the kinds of things that should move you to asking probing questions about their techniques and goals.
Look for someone who understands that rankings that don't bring real visitors mean nothing, and visitors who don't purchase or sign up mean little.
I've been thinking of writing an article on the kinds of questions that should be asked today in order to evaluate how up-to-date someone SEO knowledge is (questions about the "Florida update," "LSI"; the current situation with Yahoo's search index; what's at alltheweb and at inktomi, etc.) but haven't gotten beyond an outline yet... but there are plenty of people marketing their services who don't put the time into keeping up to date in an area where something new happens every day.
Specializing in SEO and PPC management.