If your PR is reduced at a time other than when a PR update happens or your site suddenly goes to 0 then its safe to say its a penalisation i think.
Unfortunately, there is no tool available that informs you instantly, of PR penalization... If the PR drops suddenly and stays there for couple of weeks, then that is the only sure shot way of knowing it...other wise one tends to just grope in the dark! Regards, RightMan
No Google does not notify you about this in GWT. Google couldn't be bothered to contact every website that they penalize. This would create a huge customer support backlog from people trying to find out why their website was penalized and how they can fix it. Usually a pagerank penalization is fairly obvious. If you have enough PR links that you should have PR and a PR update has passed, then you are likely penalized. You can check to see if your website is still indexed with the site:<url> command. Sometimes when a website loses it's PR it also gets deindexed.
there is not any such a tool that notify this, because there is no such penalty, your PR is low because you not have good backlinks nothing else
Google can penalize your web if you sell links or he believes you do it. In this case your pagerank is (PR value)-2