If your website has triggered a Google Penguin Filter, you will usually experience a dramatic drop in visibility i.e. organic traffic.
Now Penguin is part of Google’s Core Algorithm there are no periodic updates anymore. It runs in real-time. That means that your website can trigger a Google Penguin filter any day if you have unnatural links.
To detect potential Penguin penalties, keep an eye on your backlink profile by running Link Detox at least once a week. If you find that your site could be at risk, you should clean up your backlink profile immediately.
The work you put into analyzing your bad links and cleaning up your backlink profile will not go unnoticed by Google. According to their announcement on the Official Google Webmaster Central Blog they will see your links shortly after re-crawl or re-index.
"With this change, Penguin's data is refreshed in real time, so changes will be visible much faster, typically taking effect shortly after we re-crawl and re-index a page."
If you notice that your traffic for a specific page or subfolder has dropped, you can use Link Detox (DTOX) to analyze only that part of your website. If you find that it has a high DTOXRISK and you associate the DTOXRISK with a sudden traffic drop, then you can be sure that you’re dealing with a Penguin Penalty. It can be a partial one if the traffic drop only affects a page or a subfolder of your website, or it can be a Penguin penalty that affects your entire domain.
Don’t wait; you need to get to work and start disavowing the bad links so that you get your traffic back as soon as possible.