Take it to a firearms appraiser
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Many of the older side by side shotguns were marked on the "water table", which cannot be seen when the shotgun is assembled. If you can remove the forearm (some snap on, some have a release, some are held in place by a pin) and open the shotgun CAREFULLY- when fully opened, the barrels can be lifted free of the action. Turn the barrels over, and look for markings on the underside. These will typically include proof marking if the gun is European, and choke markings- and maybe the makers name. An appraiser or dealer that knows SXS shotguns may also be of help.