If you've got a message in Google Search Console, then it's pretty sure. If you lost 50-90% of your traffic, then it is pretty sure. If there was a Google algorithm update, then it's pretty sure...
Log in to Google Search Console. Look for messages: If Google has manually penalized your site, you’ll see a clear notification here explaining the violation and providing details about the issue.
Enter the following query into the Google search bar: site:yourdomain.com (replace with the address of your website).
but i would first look at the google search console manual actions tab to be sure. If no such information appears, then SEO performance is the problem. search which pages dropped their positions, which searches changed nd the CTR. Even if the site gets many impressions, it does not mean that it gets many cllicks.