How do you find federal reserve district on currency?

Answer:
The Federal Reserve District appears only on bills with green seals that say "Federal Reserve Note" on their front side. Older blue- and red-seal notes were not issued through the Federal Reserve System so they don't have the district.

There's a full description of the letters and districts at the BEP website:
First answer by JayKay. Last edit by JayKay. Contributor trust: 2479 [recommend contributor recommended]. Question popularity: 14 [recommend question].