Google have been really helping webmaster coming up with great way to help spot if your website has been hacked.
One great method is to use the Google Alerting system set to monitor your domain against specific keywords you do not want to be associated with. If your site starts becoming associating with unrelated queries that are popular with spammers, for example keywords related to pharmaceuticals, payday loans or fake replica goods then you need to be aware as soon as this happens.
You can set up a Google Alerts (https://www.google.com/alerts) very quickly.
An example alert [site:www.chameleonwebservices.co.uk (viagra|cialis|casino|loans)]
This will check if the domain www.chameleonwebservices.co.uk appears for the keywords requested and then Google will notify the chosen email address in the alert if this search query become active.
Google have stated:-
Monitor the Web for interesting new content, Google Alerts are email updates of the latest relevant Google results (web, news, etc.) based on your queries.
Enter a search query you wish to monitor. You will see a preview of the type of results you’ll receive. Some handy uses of Google Alerts include:
•monitoring a developing news story
•keeping current on a competitor or industry
•getting the latest on a celebrity or event
•keeping tabs on your favorite sports teams
These updates can be used how you wish and monitoring for things do not want is just as important as monitoring for things you are interested in.