Cotton has clothed humanity from time immemorial. Since the beginning, cotton is considered as the basic element of a fabric. It has found in every fabric in one way or another. In order to check the quality, just ignore the labels and tags; simply run your finger over the shirt fabric, good quality cotton should feel fine and soft to the touch. In high quality cotton, the colors are also brighter and more defined unlike synthetic fabrics.
Some high quality cotton shirts' fabric feels similar to silk. In general, it can be said that smoother the cloth, higher the quality. The smoothness of the cloth highly depends on the yarn numbers. Yarn numbers range from 30s to 200s. Usually 30s is found in cheap back alley stores, the common 80s is found in better ready-made shirt stores, while 200s is hardly found anywhere except on the most expensive high-end stores.
The 120s type is considered as the high quality and most commonly used cotton. You might have seen the shirt fabrics being described as 50s, 80s, 100s, 120s, 140s, etc. These numbers actually refers to the thickness of the yarn. Follow the simple rule that lower the number, thicker the yarn and higher the number, finer the yarn. Best fabrics are of 2-ply cotton, which is made of two lengths of yarns intertwined. Among two-plies, 120s 2-ply cotton and 140s 2-ply cotton are the most luxurious and off course expensive dress shirt fabrics.
In other fabrics 100s 2-ply Broadcloth is considered most luxurious with a silky feel. 80s pinpoint is another excellent 2-ply fabric. 120s quality oxford is also one of the most luxurious fabrics, with 20-plies and is highly durable with more texture in it.