SEO is considered the most important aspect of getting a higher page rank even today after Googles page rank upgrade has been introduced. Most web site owners/administrators either hire SEO experts or do it themselves. SEO has become so common that it seems that everyone from a content writer to a graphic designer knows how to work on SEO and how it can affect the overall ranking of your web site on Google or secondary search engines.
The first and foremost thing that you should look at while hiring an SEO expert is how well they have ranked their own website. There are many web site owners out there who would agree that they have fallen into the trap of big SEO promises at least once. Every SEO firm promises your website a good ranking on Google but how many of them can truly keep the promise is a different story. There are many websites whose targeted phrases are not even ranked forget about the ranking of the entire website or a web page.
So the thumb rule is check their website ranking before you choose to fall for their promise of SEO. There are some SEO firms who are new or dont have their website yet. For such firms, you can always check their past records...