Go into Google Webmaster Tools. If you don't have an account then create one. Send a Reconsideration Request explaining why you feel your site should not have a penalty.

Part of the confusion over Google penalty policy is not knowing what will cause a penalty. This is a very common scenario: You see good results, more traffic, more sales. But suddenly, your rankings drop precipitously along with traffic and sales. You're no longer in the search engines' index and have zero rank for all your important keyword sets. You have the dreaded Google penalty. to know more just go through it ::
Google Penalty Advice for Webmasters

You can request for reconsideration. See this link: Requesting reconsideration of your site - Webmasters/Site owners Help

How do you know it is penalised?

Doug