This article is from the Paper Money Collecting FAQ, by Bruce Giese firstname.lastname@example.org with numerous contributions by others.
Many people have had notes identified on rec.collecting.paper-money
by posting a description of the note and having the resident
experts identify it. I've even identified notes which were
digially scanned with the image sent to me via e-mail where
I displayed and identified them (e-mailing the response).
If you want to get a rough estimate on the value of a
particular note, you need to provide a lot of information
about it. Sometimes, the value of a note can differ based
on dates, color, or even the number of digits in the serial
number (although not usually). The key things to include
are these eleven things...
1: country (if you can't tell, describe the lettering as best
as possible and any features which might help identify country).
2: units of currency (e.g. pesos, pesos oro, francs, pa'anga, dollars,
dollars in gold coin, new cruzeiros, really-really-new cruzieros)
3: denomination (1? 2? 3? 5? 6 1/2? 100,000,000?)
4: issuing authority if any (e.g. Hong Kong and Shanghai Banking
Corporation, Federal Reserve Bank, The United States, Northern Bank
5: type of note, if indicated (e.g. silver certificate, military
payment certificate, interest bearing note)
6: date, if any, anywhere on note (look carefully)
7: all identifying features (e.g. portrait of bald guy lower left,
naked woman riding shark on front left, tall building on front center,
security thread in paper, overprint reading CHUNGKING)
8: colors (e.g. red and green on front, black ink on back)
9: condition of note (see section on grading notes) This is most
important since the value usually varies tremendously depending
on condition. This generally implies you shouldn't be mishandling
the note yourself.
10: signatures, if you can read them. Sometimes value depends on this.
Occasionally, the value depends on the title of the people who signed
11: watermark, if the note has one. Often the value of a note depends
on the type of watermark.