You can find out if the site has been hacked by the following methods:
1. The files contain a shell, backdoor or some other virus
2. The database is sold or provided free of charge on hacker forums and telegram channels
3. In the hosting account, vps or server, the entry from another IP address is recorded
4. Downloading of files and database is marked in the server log
5. Scripts files contain malicious code that was not there before
6. The discovered vulnerability continues to be relevant and the admin was unable to close the hole.