The easiest test is to simply touch. A good quality T-shirt will never feel like plasticky or boxy. If the fabric feels soft but firm at the same time, then chances are that it is a good quality material.
Check the labels
Most T-shirts do not have a thread count on their labels, but they do indicate the material it is made of and if it has been mixed with something else. Care label indicate how a particular products should best be washed, dry cleaned or ironed.
Make the wrinkling test
Scrunch a part of the garment in your hand and then release. If it retains a lot of wrinkles then it is possibly bad quality, if it doesn’t retain any wrinkles at all then it is perhaps a synthetic material. Ideally, you’d be looking for something between the two.
Check the transparency
Handhold the garment against a light source and see how transparent it becomes. The more transparency, then the less density, less transparency means more density and good quality.
Stitches are the muscles of a garment; they’re what keep everything together. Because manufacturers will always pay more attention to the outside of a T-shirt, the easiest way to tell if a stitch is done well or not is by turning it around. Is it even? Does it lie flat? Are there any loose threads? These are all signs of poor manufacturing. Also, check out how many stitches it has. The higher the count then the better because it means that more time was spent putting this tee together.
Like stitches, hems are revealing of how much care was put when manufacture the garment. When it comes to T-shirts, you should be paying specific attention to the collar, the sleeves and the lower hem.