A true SEO expert is never supposed to perform such activities that could lead your website to permanent removal from the search results.
The obvious way to detect a penalized site is when it gets blocked and has been penalized with termination but this is only for fewer websites, in rest of the cases one can check the site being penalized with useful penalty tools. Sometimes Google also sends a note via a registered mail or webmaster tool to notify the site owner about the negative strategies used in your website. If immediate actions are taken, to recover from such penalties further terminations can be stopped.
Google penalty is a punishment given by Google to those websites which have used negative SEO techniques for their website rankings in Google search engines. The basic updates responsible for this are Penguin & Panda by Google. Sometimes websites are also punished because they are not mobile friendly. In simple terms that websites which according to Google are not up to the mark or have negative strategies are penalized.
The first and initial step to recover from penalties is its detection. Using tools one can know if their site has been penalized or not. Once it is identified that site has been penalized, it is also possible to find out the reason for this. One can make changes and correction for those reasons responsible for such penalties. If Google has sent you a note you may work on it and can send back reconsideration request to Google.
Content is the key to success & Google is extremely serious with this. Hence a low quality, poor or duplicate content can hurt the website rankings.
When a website uses inappropriate keyword stuffing strategy to achieve faster results. The Google update penalizes the website instantly.
More than the quantity of backlinks, quality of backlinks are counted by search engines for ensuring better ranks to any website.
When a website tries to fool the search engines by showing different page for the search engines and another page to the search engine users.
Once you find a best tool to find out that is your site penalized or not you can take these necessary actions for correction of all issues such that you can get your website live back again
These are the obvious reasons why site is penalized there can many other reasons which are as per Google algorithm. It depends upon the penalty that your website is going through if it is an algorithm penalty there is no need of sending a request for reconsideration because Google will resolve the issues by itself once you have made necessary changes. In case manual action is required to be sent for reconsideration, one can send reconsideration to Google.