It will be interesting to see how people respond to your question!!
For me, the difference is in how it feels. It should feel "substantial" -- not limp, not so thin you can almost see through it. Partly a matter of threads per inch and the kind of sizing it has, I think. Perhaps also in the weight of the thread used to weave the cloth.
Also, is the color "registered" correctly (are there shadows, or ghost images that result when the colors are not lined up correctly?) Are they 'saturated,' meaning that the color -- whatever it is -- is clear and deep. Even if the color is a pastel or lighter color, you should be able to see that the color is even throughout the area you're looking at.
It would be easier to show you examples of good quality fabric as compared to lesser quality fabric. Not sure picts would be adequate, though.
Jory