Whilst it may look like we're trying to punt our own SEO service, this article is written as a honest aid for anyone in the market for an SEO provider - here's what you need to ask, know or find out when "shopping" for a SEO company:
Get A Guarantee
Just because SEO is a complex, inexact science does not mean that you as the customer must assume the risk. If you're going to be handing over your hard earned money to an SEO company, make sure that they specify and guarantee certain deliverables - these could be guaranteed rankings, backlinks, or traffic. Whatever it is, make sure you have some form of guarantee, or you will run the risk of getting burned.
Get An Explanation
Although SEO companies generally won't list all their tactics in the actual quote (if they did the quote would be rather long!), they should be able to explain to you, in plain language, what they'll be doing to improve your rankings. If any part of that explanation smells fishy, simply Google it for an unbiased answer.
Get Some Examples
Forget references (they can be underhandedly paid off) - ask for some evidence of real work done. Any SEO company worth their salt should be able to provide at least three websites for you to Google their results. If not, you really need to start heading in another direction.
Avoid The Cheapest (and the most expensive...)
While their are some excellent SEO providers out there who charge a lot less than they're worth (due to any number of reasons), chances are that the cheapest is not going to be the best. The same applies for the most expensive SEO company - chances are that your money is fueling their marketing... Do your due diligence.
Never Link Back
Any SEO company that requires you to link back to their website as part of the deal is simply trying to build their own backlink portfolio on your time and money. Be sure to ask about this, as many companies won't even tell you they're going to do this beforehand...
Check The Reputation
Does the company have a reputation outside of its own website and SEO efforts? For example, are they featured in business magazines, publications, etc? Do they contribute to the online community, etc? The more effort they put into building their reputation online and offline, the less likely they are to be "fly by nights".
These are just some of the important considerations to make when looking for a SEO company. Most importantly, don't fall for the hype and big promises of online success. Do your homework and choose the company that offers the best value to you and/or your business.
Questions, additions, etc welcome