The easiest way to get something like this valued is to take it to an auctioneer. (Simply say that you're thinking of selling it and would like to know if it has any great value).
A quick look in Yellow Pages should enable you to identify local auction houses that deal in fine arts. Even the 'big boys' in the field, like Sotheby's, Phillips, etc, are usually happy to give 'on the spot' valuations free of charge.
I've often used the services of a local branch of Phillips. I've also turned up, without an appointment, at Bonham's in London and been given a great deal of time by one of the guys who appears on the Antiques Road Show.
Similarly, I've taken a friend to get some items valued at one of Sotheby's auction rooms. In every case we've been met with courtesy and consideration and received valuations totally free of charge and without commitment.
Chris