If you can read the model # or serial number you can figure out when it was made. Then do a search on the net for that exact gun. You can probably find a approx. value that way.
It worked for me when I inherited a semi-auto colt made in 1904.
Register on the Colt Forum, post up lots of good pictures in the Percussion Revolver section and you will get good information and possibly some offers to buy it.