When trying to find an SEO agency it can be very difficult to establish a list of criteria especially when you don't understand the SEO process. Almost every SEO consumer would expect a good SEO agency to appear in the top of the major search engines for all their relevant key phrases. Although this concept seems very logical it doesn't necessarily reflect upon the other agencies which aren't. There are literally thousands of SEO companies out there and with a relatively small number of positions on the first page, it's impossible for all the agencies to prove their value by being on the first page. It's only when you start to break down the current high performing SEO agencies that you start to develop a better understanding as to why they are there and most importantly why you should not use them. There are endless checks you can do on an SEO company but I would strongly urge anyone to carry out the ones listed below followed by any further checks you believe to be suitable.
Search their company name
Searching their company name will inevitably bring up their directory listings, article submissions and social networking pages but if the company is relatively large, there will be some feedback somewhere. It's highly likely if someone has had a good or bad experience with an SEO company, they would have mentioned it on a webmaster forum or a blog. It's very important not to take these reviews at face value because some unethical SEO agencies promote their services this way, if it's a forum look at their post count, join date and response from other forum users.
Look at their portfolio
If an SEO agency is good at what they do there will be an endless supply of testimonials, don't be afraid to ask the agency for previous examples of their work. If an SEO company advertises a large amount of clients but yet only displays a set few of testimonials, ask them why or even better ask the owners of these websites. Webmasters are more inclined to tell you what's not working or hasn't worked as opposed to what's working well.
Check their backlinks
A large proportion of online marketing is backlinking and there are endless amounts of free tools to check the backlinks of a particular website. The quality of the websites linking to this SEO Company will reflect upon the ethical standard of their work. If you find an agency to have a vast amount of backlinks from websites you would consider as being spam it is highly likely this is the approach they will take to marketing your website which is against all major search engine guidelines.
It's very important to know how an SEO company will work on your website if at some point you believe you are being blinded by technical jargon or if you feel as if the company is not being completely honest it's highly likely the agency is working unethically or in SEO terms using Black Hat SEO. If at any point you discover Black Hat SEO I would strongly urge you to leave that SEO Company.