Discerning people pick up on subtle traits and are good judges of quality — they're the ones that can tell if your cupcakes are homemade from the finest ingredients or totally from a box mix.
Discerning is an adjective that comes from the Old French discerner, meaning to “distinguish (between), separate (by sifting).” Which makes sense, because someone with discerning tastes or a discerning eye is good at distinguishing the good from the bad and sifting out the gems from the junk. If you're an ace at picking out fabulous fabrics, accessories, and shoes when you get dressed each morning, you probably have discerning fashion sense.
adj having or revealing keen insight and good judgment
characterized by careful evaluation and judgment
characterized by ease and quickness in perceiving
clear-eyed, clear-sighted, perspicacious
mentally acute or penetratingly discerning
perceiving the significance of events before they occur
showing or indicating careful judgment and discernment especially in matters of taste