When garments are cut and fit poorly, the first thing you may notice when you put it on is that it is not shaped to fit a body like a body is shaped. Try to move around and twist your body. If it doesn't have room and flexibility to move, it's likely a poor fit and cut. There are a few contributing factors in the manufacturing process that influence this.
1. Fabrics/Fibers: Try to push the loops of the weave of fabric back and forth. If you can do it, it's too loose and usually represents a poorly made product that won't last. If you're shopping on line, look at the weight of the fabric on the model and you'll know. If it falls softly around the curves, it's usually good.
2. The cutting process: Fast fashion manufactures don’t care which direction they cut the fabric, they just want to optimize all the fabric. So, you will often see seams that twist more and more with every wash because the fabric was not cut correctly.
Cotton is a much more comfortable product than synthetics, however, 100% cotton products are not a guarantee for softness. Cotton production has been so optimized that really cheap cotton can be found and it is just not as good of a quality.
3. The stitching: A hem thread should be flat on the fabric, not bulky or overlapped. The inside stitching should be clean and straight too. A well made product will have pocket seams that are hidden. Cheaper stitches are meant to be temporary and will fall apart after a few wears and washes.
4. Color: Cheaply made products are not dyed correctly and will fade quickly with washes. If the wash instructions that say, “Don't wash”, it's a bad sign. For sure the color will bleed and ruin the product, but even more so, how does "Don't wash" work for your life?