Even if you don't go to school for fashion, there are a few easy ways to learn about the details. Most people live near department stores, and if you're lucky, there's probably even a high-end boutique you can get to. Compare pants and shirts you own to the pricier stuff, and really try to notice the differences in cuts and design. Turn those same garments inside out to see how they're made. Chop it up with different associates who seem to know the product, and learn what makes it "good." As you see more and more clothes from different designers you'll know your Balmain from your Balenciaga without having to look at the tags.
Learn about fabrics, garment construction, and fit