The FRN is also listed on the FCC license search results page (
License Search). Just search on one of your call signs and you can bring it right up, quickly and easily (with minimal brain functionality needed!).
You can even search by FRN if you know that and want to check what other licenses you have (and may have forgotten about). When searching by FRN, be sure to enter all 10 digits, even though most often the first several are zeros. The search is character based, not number based so 0000123456 is totally different from 123456. If you copy and paste, make sure that you don't have any leading or trailing spaces (or other characters) since those will also cause the search to fail.