How to choose your SEO professional carefully?
Unless you're willing to give an opportunity to a beginner, you should require concrete and recent references.
Keywords ranked on Google's first position, number of visitors generated coming from search engines, etc.
Don't get too impressed by a good ranking on Yahoo and Bing because it is sometimes easy there to get impressive ranking on keywords that actually generate no traffic.
A good SEO service provider must be capable of explaining you clearly their strategy and working methods.
Transparency is the watchword. The veil of natural referencing secrecy has been lifted. In case the explanations are confused, run away.
You must also have as much visibility as possible on the time spent on your project as well as on the completed tasks.
It is essential that you require receiving information on set dates about the evolution of the project and the results (visits, ranking...).This will allow you to evaluate the benefit of the service.
Beware of the guaranteed results, you can't ask the impossible with a limited budget. Watch out for overconfident providers, too. SEO is rarely an easy task and no one can know what Google's final choice will be.
Remember that there are only 10 positions to take in first page (or even 9,8, 7 or less) and probably 20 competitors already with 20 SEOs involved in the battle have possibly more budget than you.