1. The seams aren’t straight or have loose threads.
One of the fastest way to determine the quality of an item is to turn it inside out and look at the seams! If the seam has loose threads, is not straight, or appears to have been stitched over multiple times, the item is not good quality. You can also grab the fabric on either side of the seam and gently pull it apart a little. If the fabric at the seam separates, the seam is very weak and was most likely poorly sewn.
2. The fabric is mostly synthetic or poor quality.
First step, read the fabric tag. Ideally, you would want the fabric to be mostly natural fibers (cotton, wool, angora etc.) rather than synthetic fibers (polyester, rayon, viscose etc.). There is a big drop in quality when you drop below 80% natural fiber. Fabric with mostly natural fibers and somewhere around 1-5% synthetic fibers (particularly lycra and elasthane) is more likely to retain it’s shape without getting baggy over time. Also, viscose is one of the fastest synthetic fibers to stretch, pill, and generally fall apart. So don’t buy viscose if you want to be able to wear the item over and over.
Second step, inspect the fabric. Run your hands over it and look for flaws. Grab a section and crumple it in your hand for 10-20 seconds and then release. If it retains tons of wrinkles, steer clear. If it retains no wrinkles at all, steer clear. Hold the fabric up to the light and stretch the fabric to see how tightly woven it is, and how much give it has. A thin low quality cotton will have lots of give and let through lots of light, in comparison to a higher quality cotton.
3. There are no facings or interfacings.
If there are no facings or interfacings, the item will quickly get that rumpled sad baggy look that cheap items get after a few wears. Interfacing is a strong fabric sewn in between the layers of a garment to help it hold its structure. Facing is a piece of fabric inside a garment opening, like a sleeve or neck opening, that enclose the raw edge of the fabric. Both facings and interfacings help a garment hold its shape. You especially need them on blazers and skirts/pants. There should be interfacings around the waistband, by the buttons, on the collar and shoulders, at the edge of the sleeve, and on the hem. You can see facing just by turning the item inside out, and you can tell whether there is interfacing by feeling around where it should be. Cheap clothing manufacturers will often skip facings/interfacings to save money, and the garment will look just fine on the hanger, but it will quickly become misshapen after several wears.
Well ok, not all articles of clothing need a lining, but the kinds that do need one really really do need it. Linings seriously prolong the life of an article of clothing by providing some slip when you put a garment on or move around in it, which minimizes stretching of the garment fabric. This keeps the fabric from getting misshapen and baggy, especially in key movement areas like the elbows, knees, and butt. Linings also protect the outer shell from sweat and skin oils. If you buy a skirt that doesn’t have a lining, you can actually accomplish the same thing by wearing a slip underneath the skirt.
5. The buttonholes are flimsy.