The five most important questions i think you need to ask when looking for a seo specialist are:
What achievements / accreditations do you have? (any good seo specialist will have a few accreditations maybe a book regular spot on an authority site)
Who have you worked with, what results have you achieved for them and can i talk to them? (if you cant see any past results or talk to clients be very wary)
What search terms do your site rank for? (a top search specialist will be able to compete on a global stage and compete with other specialist and this is a good test to see how they really are)
Do you offer any guarantees? (if they say yes hang up there are no guarantees as its up to the search engines refer to there history or achievements)
When should i start seeing some results? (you should look to see improvements within a month but you will need to be patient as it can take a long time to achieve top placements for very competitive terms)
Very basic, but essential and if you ask the above questions you are more likely to make the right decision and hire a good SEO Specialist.