If you're booking directly with the airline, there can be airline-specific ways to see the fare basis code, such as going to advanced search if available or starting to purchase the ticket, then clicking to see the fare rules (and therefore the entire fare code, which often can telll you the fare class as well as other information about the fare if you know how to decide it for that airline.

Another idea is to check fare class availability on EF. If a fare class isn't available (for instance, when searching on EF, you see I0) there's a good chance that your ticket won't be that fare class (I in this example), although married segment availability can mess this up. Also, the fare class can be available, but there might not be any applicable fares due to restrictions such as advanced purchase or minimum stay requirements, although this is likely to lead to a higher fare class, which is better for earnings.