You need to start by knowing the exact model number. Or at least very close!
Go to the S&W site for starters to determine the model if you don't know.
(You can't just come on and say "black with 7 rounds")
Once that is established, I go to a site like GunsAmerica and find a gun like mine to see what it might sell for, market value at least.
http://www.gunsamerica.com/You can also brouse there to find your gun/model.
You can also go to a book store and go through one of the many gun "Blue Books". But they don't aleays give you an accurate market value: that which it would sell for.