How to tell if you have been hacked.
It may be obvious. The hacker may have loaded their own version of your Home page, just because they want to show off their skills.
If there is nothing obviously wrong with your website, then you can check for known hacking vulnerabilities on sites such as sitecheck.sucuri.net. This tool will check for malware and will also check your site against various blacklists maintained by security software companies such as McAfee. If the scan comes up clean then you can be fairly confident that you have not been hacked.
How does a hacker get into your site?
They may have gained access to your web hosting account by some means which would then allow them to publish new pages and modify existing ones. Alternatively, they may have found a way to upload their own hacking scripts via a vulnerability in your site.
