How can I tell when my fat is sufficiently creamed? When creaming butter or shortening with sugar, how can I tell when it's creamed enough?  

With the mixer as low as 3 or 4, even a few minutes of mixing seems excessive.  

Also, what are symptoms could I see in cookies which have had the butter under-creamed?



Update:  

I think that I'm familiar with the standard guides of light and fluffy, but I seem to have a hard time evaluating that when I'm watching the mixer.  My cookies always seem to come out too flat, and since I'm measuring flour by weight, creaming is my next suspect.

What should I look for?  Are there any tests that I can do; perhaps similar to the window pane test for bread development?
The mixture should lighten in both color and texture, and it should be 'fluffy'. It should also increase in volume.

The point of creaming is to incorporate the sugar with the fat, while at the same time adding air to the mixture. The air bubbles introduced during creaming expand during cooking, making the cookies rise and giving a lighter texture.

Cookies baked with under-creamed butter would therefore remain fairly flat and dense.