In October 2012, Google was said to report “practically 100%” manual actions. This happened around the same time that the Google Penguin 2.0 algo was rolled out, and for Google, those notifications became a major instrument to let webmasters know about their penalization.
Before that time, it was often a lot of guesswork for SEOs to try to detect a penalty by
using the SITE: command to spot de-indexed websites (Google has since changed the behavior of this and other SEO operators)
checking if the Green Toolbar PageRank bar was still showing some green or was gray (which often indicated a penalty)
Comparison to Google Filters
Even if there is no “official” Google Penalty applied, a website can still suffer from suppression in search results due to a Google Filter, which is not reported and often reason for a lot of guesswork and discussion among SEOs.
Special Case Google Penguin
Google Penguin started as a harsh penalty issued by Google in 2012 and was transformed into Google Penguin filters with Google Penguin 4.0 or “Real-Time Penguin” in October 2016.