The page loads conspicuously slowly
Going to certain menu items the site will redirect to other web addresses, unknown ads may appear, unknown pop-ups may appear
Emails are sent to SPAM, ranking is suddenly dropped by search engines
The browser warns you that the page contains malicious code
The hosting company closes the site (their internal virus scan finds a virus)
The site loads completely different content (e.g. bank login interface)
The website displays strange error messages, typically around the footer
Google Search Console sends a warning message
IT Security Company will alert you by email and blacklist the page or certain parts of it
There are new (admin) users in the database