The obvious signs are sudden redirects, pages you didn't create, warnings from Google or your browser, and users or files you don't recognise. The subtle signs are drops in traffic and search results pointing to strange pages.
The obvious signs
Visitors get redirected to spam or gambling sites. Google warns people away from your site, or marks results "this site may be hacked". New admin users appear that nobody created. Files in your hosting account have recent dates you can't explain. Your host emails you about malware.
The subtle signs
Traffic quietly drops because Google has quietly de-ranked you. Search results show odd page titles. Your site sends email nobody asked for and your domain ends up on spam lists. The site feels slow for no reason. On the inside, plugins deactivate themselves or errors appear in the log that nobody can trace.
What to do if you suspect it
Don't delete anything yet - you may destroy evidence of how they got in. Don't restore a backup until you know the backup is clean. Get the site scanned properly, by someone who does this every week. Our Hacked Site Recovery page walks through the full containment, cleanup, and hardening process.
Rather just have it handled?
Hacked Site Cleanup
From $249, flat rate. You approve the price before we start.