There isn't an easy or accurate way to do this. You can use systems such as alexa, quantcast, and compete to get an idea. (lookup your site in each service and see what the "related" sites are)
While that's true with alexa, the other services list the sites that your visitors do visit. As I mentioned, there is not easy & simple way to do this. Especially for sites with low volume (accuracy & completeness for these services is much lower)