Are you on a shopping trip or armchair shopping?
An artist friend of mine gave me a hint several years ago. She uses her fingers as a "frame" and only looks at a portion of the fabric. That helps the eye to focus on the colors and design. Sometimes looking at the entire piece is too overwhelming and you miss a detail or color.
I think this is probably similar to looking through a door peephole, without the peephole. It works for me... every time!