Many hear tales of great thrift store victories--designer pieces scooped up for mere dollars, but when faced with rack upon rack of poly-blend and elastic-waist jeans, they despair of success. Thrift shopping is more competitive than your standard department store experience. There's usually only one of something, which means you need and edge. The best hunter is the person who is at one with her environment, who can quickly detect the signs of quality prey. The thrift store can be a rather chaotic shopping environment. Understanding two key signifiers of quality will help you to quickly ferret out worthwhile pieces. Armed with these principles, you will rise above the chaos, extract the gems, and be on your way.
Fabric and Construction.
With fabric, you really only need to ask yourself one question: is it rough or soft? Answering this question should tell you if you're dealing with a blended fabric. (If rough, then "yes," if soft, then "no.") And it will, for instance, tell you if the fabric has a high thread count. (Higher = softer.)
With construction, if you're looking for suits, do not settle for anything that isn't lined. A good suit is like a suit-of-armor. The magic is in the drape, and the drape is due to the lining. Don't go into battle unprotected!
A few hints I picked up from Kathryn Finney's How to Be a Budget Fashionista, on seams: was the fabric dyed before or after the piece was assembled? A thorough tailor will respect the fabric and dye it before assembling the piece. A thriftier tailor will dye only the fabric that ends up on the piece. Hems and seams should always be sewn and not glued for durability.
Is the detailing consistent on front and back? If the embroidery doesn't continue to the back, don't buy it. You are a three-dimensional being!
That's really all there is. You need to practice recognizing these indicators of quality, until you briskly walk the aisles, not feeling, but seeing the softness. Happy hunting!