In the TV show "Project Runway" they pit a bunch of fashion designers against each other. At the end they judge the clothes that they've made. One of the more positive they can say about a piece is "It looks expensive". The trained eye can quickly ascertain things like that (the material, the cut, the stitching). One of my friends (she works in fashion) says she can immediately tell if a guy is wearing high-street brands (H&M and the like).