It's mainly getting a feel for the fabric. Does it feel thick and durable, or thin, stretchy and brittle? Are the seams sewn straight and evenly? Hold them up to the light and see if the light shines through the fabric evenly, or if there are thin patches, this is likely where the item will tear. Like I want t-shirts that wash well and wear out evenly, so they get that super soft feeling as they get older. Has the fabric aged nicely, or is there a lot of fuzzballs and pilling, especially on synthetics, which might also suggest the previous owner didn't care for it properly (like washed and dried it at too high of a temperature). If it's something that is supposed to have some stretch like athletic wear, you can sometimes see that the threads of the stretch material have snapped like little broken rubber bands, the fabric should look uniform and smooth. Check how it's supposed to be cared for, personally, I am against dry cleaning from the environmental impact, but it's important to look at how it's cared for. Older clothing often required ironing, starching, and pressing, but there was also permanent press which usually didn't wrinkle as easily and had pre-set seams. Now clothes are more wash and wear friendly. Will the garment care require more time than you are willing to commit to it?

I will lay clothes out, fold them multiple ways, see if the garment is symmetrical and the seams line up. Lower quality clothing from overseas, it amazes me how uneven they are, no offense to the people who made them. Older clothing may have a union label or logo on the tags. There might also be quality control or inspector numbers on the tag, which means someone actually hand inspected the garment for quality. Prior to about the 1980's, a lot of clothing was handmade, and I've found those clothes to be superior quality, since hours of work was usually put into making a single garment. So they'll feel very well made but not have any identifying tags.

Also with thrifting and BIFL on a budget; I learned how to hand sew and am trying to learn how to use my sewing machine so I can tailor and alter my clothing. Nothing makes a garment look more flattering than tailoring it to you, especially dressier or business attire and jeans. It's also helpful to repair things like buttons, zippers, and patch items. Upscale department stores used to have an in-store tailoring service. You would buy a garment, know your measurements, and the tailor would personalize it to you. Now, you can learn to do it yourself and give your clothes a very customized fit. It's good if you find an item slightly too large, you can take in the seams a bit, or add darts, for example.

You also want to look up tricks to identifying and removing different stains. Most food stains can be removed, but I'd avoid anything that looks like mold or a possible bodily fluid. Yellowing of white fabric is also often permanent, but there is a product called bluing that will tint whites slightly bluish to make them look a brighter white if the yellowing/dingy color is only slight. Sometimes hanging them outside on a sunny day also brightens fabric, and remove some odors that washing didn't get out. Maybe it has been in a dark closet for too long and picked up a musty odor, but airing it outside can help. Heat from the dryer will often set in stains and odors, so if one wash didn't get it out, try washing it again before drying it.

It's helpful to go to the thrift store often, know when they put out new items, sometimes expect you'll go home empty handed. It seems like thrifting gets almost competitive during economic recessions.