New - How Do I Determine Value Of A Stamp

I am not a stamp collector but do have some cancelled stamps on envelopes that are first day of issue. I just happened to be in Baltimore, MD, in 1983 or 1984, and visited the Baltimore Harbor. There was a long line and a big uproar over a stamp that was being issued that day as a joint effort between the U.S. and Russsia. I stood in the line for hours and purchased several of the stamps. Some of them are marked canceled in English and some of them are marked canceled in Russian.
Now it is twenty years later and I am needing guidance in determining if these stamps have any value. The stamps are of sea mammals and dolphins.
These are the only stamps I have and don't have a clue as to how to go about this. Can anyone here give me some advice on how to proceed?
Thank you.

That sounds like the 1990 set; the U.S. stamps are listed in the Scott catalogue as ##2508-2511.

The market for first day covers (FDC) is not strong right now. I would check ebay or bidstart.com, searching for "US 1990 joint issue" or "US 2508" to see if you can match up your covers with existing listings. You will want to make sure you're checking for the same envelope design.

Once you have found something that matches what you have, check completed listings to see what actual selling prices have been. Right now, on bidstart, you can buy a set of all four U.S. stamps from this set on FDCs for a little over $2.00, total.

I am very grateful for this response as I now have a blueprint on how to begin this search.

Thank you.