Most of the time, when a domain is penalized by Google, it disappears from the google search engine. If you type “site:yourbannedwebsite.com” and if you see no result in google search engine, it means your website is banned from Google. You should see something similar to the following screenshot.
However, I have also noticed stealthier bans where the website was still indexed by Google and when the traffic tremendously dropped. Then, you need to investigate a bit further.
In google webmaster tools, if you see a drop in impressions in the course of 3 days to 1 month and if your website is not seasonal it means something goes wrong. Your domain is likely penalized by Google.
On the other hand, a sharp drop in impressions may mean that there is a technical problem with your website and you should check the reachability of your pages.
Here is an example below of a domain penalized by Google in Google search console.