Excellent post called Has "Your Site Been Hacked? Here is How to Find Out" on Stepforth. It’s really quite simple, use the Google Toolbar to re-translate your site back into English and if it looks different than the way it normally looks, it’s been hacked.

Here’s the whole thing:

Identifying a Hacked Site
So how can you tell if your website is being affected by this cloaking hack? First of all Matt and his host Vanessa Fox (also from Google) goes to great lengths to note that this is a very rare hack so keep that in mind.

Here is the simplest way to discover whether your site (or your competition’s) is using cloaking:

1. Ensure that you have the Google Toolbar installed and using the options button within the toolbar enable the tranlation option so that it appears on your toolbar. Note that you may need the newest version of the toolbar to do this (I am not 100% if it was added to previous versions).
2. Visit the website you want to check.
3. Click "Translate" (or "Translate into English" depending on your browser) and the page will be instantly translated into English from English.
4. If what you see is different than your live site, you had better contact your webmaster immediately because your website might have been hacked.

How Does this Technique Work?
This translation process beats the anti-detection technology used by cloaking software so that you can see what Googlebot actually sees when it visits your website. This is a very effective method of testing your site but don’t forget to check your competitor’s as well. After all, you may find out that your competitor is purposely using cloaking to get ahead in search engine rankings. Cloaking is fiercely prohibited by search engines so you can expect your competitors will not get away with this for long. If it turns out they have been malisciously hacked I recommend letting them know, however it is ultimately up to your conscience whether you will inform them or not.