You wake up one morning, check out your site statistics and learn your traffic has plummeted. You then dig deeper to see the source of the traffic drop is from Google sending less traffic for your main keywords. Most SEO companies have gone through the experience of having at least one site penalized. The question is how does an SEO company first confirm a site penalty and then remove the site penalty.
We will discuss a thread at WebmasterWorld that has SEOs offering advice on this topic.
One of the sure fire ways to determine if a site was penalized by Google is if the site no longer ranks for it's own domain name. So if your site's domain name is rustybrick.com and you type in rustybrick into Google and you don't come up in the first page, then you got pretty good evidence that you have been penalized.
Also, since you have statistics on which keywords you ranked well for. If most of those keywords are now ranking in the 5th plus page, then that is often a sign of a penalty.
Senior member, johnnie, suggested that you first see if it is a malware issue. To do so, either go to Google Webmaster Tools and login (make sure your site is verified) and it should tell you in the messages. If not, then go to google.com/safebrowsing/diagnostic?site=mysite.com and replace "mysite.com" with your domain.
But I prefer Google Webmaster Tools cause they may show you other issues with your site. It doesn't always show you if your site has been penalized, but it is a good start.
Here is a good checklist of items to review to make sure you fix your penalized site. We also wrote about how long sites are penalized in Google. In fact, we have lots of good articles about a Google penalty in our archives.
Forum discussion WebmasterWorld.