Latest Coin World has an article that points you to a site with a pictoral (and verbal) description of FBL for Franklin halves. IMO well worth bookmarking. It's in the CAC website. www.caccoin.com Then look for "Franklin Half Dollar Reference Set" on the right.
Here is a better site with a great description. http://boards.collectors-society.com/ubbthreads.php?ubb=showflat&Number=2426204 I'm not affiliated with the site, but enjoy their approach on FBL.
I've read it, and to be honest, I still look at the set of BU franklins I bought right out of mint sets and couldn't tell you if they are FBL or not.