Do a search on Google? Sounds a trite answer but that is the only legitimate way of doing it according to Google's T&Cs which warns against using automated tranking checkers. Why do you need to check rankings? If you have any form of tracking software you should be able to see which keywords rank by seeing which ones are delivering effective traffic.

Try this out...
This will determine how a page is targeted based on its keyword. - [seomoz.org...]