Google have updated their Webmaster Central service to alert you of spam violations in your website. You will be notified of pages that have been removed from the Google index, the reason why, and how you can get reconsidered for inclusion.
The new Message Centre in your Webmaster Central account will contain correspondence from Google in the event that Google has spotted some hidden text or other violation on your website.
Currently, you need to be a verified owner of the site and logged in to your account in order to receive this information. Google are planning to resume sending out email notifications shortly.
The spam violation service had been around since 2005, with notifications provided via email. But with spoof messages flying around, Google decided to halt the service.
Once you have made any necessary corrections to your site, then you need to go through Google’s Reinclusion process. I think it is well worth noting that Google asks what search engine optimisation (SEO) company you are working with that led to the violation:
Tell us more about what happened: what actions might have led to any penalties, and what corrective actions have been taken. If you used a search engine optimization (SEO) company, please note that. Describing the SEO firm and their actions is a helpful indication of good faith that may assist in evaluation of reconsideration requests. If you recently acquired this domain and think it may have violated the guidelines before you owned it, let us know that below. In general, sites that directly profit from traffic (e.g. search engine optimizers, affiliate programs, etc.) may need to provide more evidence of good faith before a site will be reconsidered.