It's interesting to note that identifying a great SEO agency has always been a problem for many. Either in the past or present, the issues still seems to be troubling a number of small business owners who would wish to grow their brands.
To learn how you can separate the good from the bad check out at their pricing.
Good clean whitehat SEO requires some investments, so the cheaper the service the more likely it is to turn out bad.
Demand to see their track record.
Put them to the test and evaluate if you still want to work with them.