I generally use Yahoo! Travel for this. Go to the travel section of Yahoo!, select the 'Air' icon. It will take you to the air search page. Put in your origin and destination airports and select the 'Find Best Fares' button. (if you'd like to limit it to a particular airline then scroll down a bit and select to search certain airlines but still use the 'Find Best Fares' button above).

It will give a listing of the fares. Using your mouse, position your cursor over the 'rules' link associated with each fare. At the bottom of your browser in the information window (thin grey line along the bottom of the browser) you'll see the details of the fare including fare basis code.

[This message has been edited by tfjim (edited 07-20-2001).]