I buy pretty much all my clothes (and some for my husband & kids) used. I mostly haunt thrift stores, but occasionally go to consignment stores, and will stop by a garage sale or two a few times a year. I select clothes to try on by the feel of the fabric, then by name brand. I believe the quality of Brooks Brothers, Nordstrom in-house clothes, and to a lesser extent REI to be superior to other, comparable garments. I do look at seams, but find that small holes, stains, and pilling are the most common problems I find with thrift store clothes.
As an strategy for increasing my efficiency in quickly rifling through the huge quantity of clothes, I do scan for brands like Banana Republic, Polo, etc. I do this not so much because the quality is markedly higher than less expensive brands, but because the style and fit are generally nicer. Because I almost never go to the mall, I find I am not that familiar with many popular brands. I have discovered a brand I really like called Royal Robbin. The quality is great, althoug I don't know where it's sold, and I get excited anytime I run accross it in my thrifting adventures.
It is obvious to anyone who has spent time in thrift stores, but there is a huge surplus women's clothes in good condition & a much smaller supply of high quality men's clothes in pristine condition. Especially for men, if you don't enjoy spending time in thrift stores, you may be just as well off buying a few nice pieces at a place like TJ Maxx or some equivalent.