A brilliant idea codes from Google’s “Official Google Webmaster Central Blog“. If your site gets hacked you might not notice it until it’s to late. And it can be a lot of work to check your site each day.
So Google suggests to use Google Alerts to scan your site on a regular base for common malware terms by searching for something like “site:example.com viagra OR casino OR porn OR ringtones”.
It’s not a 100% solution but I’d say it’s pretty good and quick to set up.