Take to forearm off and remove the barrels from the frame. There should be proof marks and serial numbers etc stamped either on the underside of the barrels at the chamber end or on the watertable (the flat area of the frame that the barrels sit on). Once you find the serial number you can cross reference with this chart to give you an idea on when the gun was made. Post pics as soon as you can!
http://www.doublegunshop.com/dgsnos5.htm