Good question.
Check this out:
http://stamps.net/how.htmYou can also look up stamps on stampsfinder.com or
freestampcatalogue.com and try to find your stamps
and any possible current prices. A Scott's Stamp Catalogue
at your local Library is also a good place to check out values-
call the library first--I have found a few local libraries
that do not carry Scotts for stampers.
Remember that the condition of the stamp effects the values-mint
versus used, any creases, missing perforations along the edges
of the stamps, etc. Or do a serach on this site and see if any
posts have been made on the topic you are interested in. Don't forget--you
can always ask someone here,too.
Gussyboy1