It’s not hard to tell the difference between Tilgner’s Ruby Red Olde World Scottish Style cold-smoked salmon ($85 a pound at Shelsky’s Smoked Fish, 251 Smith St., nr. Douglass St., Boerum Hill; 718-855-8817) and your everyday lox. There’s the striking hue, the lean but succulent flesh, and the assertive flavor, a result of dry salt-brining and then cold-smoking the wild sockeye over alder wood. When a customer of Peter Shelsky’s brought him a sample after an Alaskan fishing trip, the appetizing-shop owner promptly placed an order, making him New York’s exclusive retailer. Local aficionados are already calling it the Benton’s bacon of smoked salmon, and that might be an understatement.