FEEL the fabric, obviously not all 100% cotton is created equal and if you feel it, you'll quickly notice the difference. Also, if it feels rough, it will probably still be rough after washing.
Those really expensive Egyptian cotton sheets, which feel like soft butter against your skin, are expensive because of the thread count.
And if you hold fabric up and really look at it, often you can see how tightly, or loosely, the threads are woven.