The Three Tests of Drawing

'ware the three trials of the draw!
Sometimes when drawing, something may just not look right. These three tools are a great help for finding just what it may be:
The Thumbnail Test
Zoom out of your illustration and ask yourself if it looks good. If a drawing doesn't look right in a small scale, or from far away, it's probably not good enough. In Photoshop, using the Navigator window is a good way to have a constant thumbnail look at the drawing.
This test should be checked at all stages, sketching, painting, detailing.
The Mirror Test
Flip your drawing, and have a look. You may be horrified. A drawing my look OK when you work on it for a while, and after being immersed in the drawing you may not see problems, but by flipping it, you may discover some problems you haven't noticed before. Make a habit of flipping your drawing every once in a while, you may even like the other side more!
This test is best checked at the start, it may save a lot of headache later. However, don't be afraid of using it later too, remember, you're drawing to improve, don't be afraid to see problems.
The Value Test
Look at your image in grayscale. Ask yourself if the overall value in the image is good, and see if areas are too flat or the image is too gray. Look at the shadows in some parts and compare them to other parts. Colors may be confusing. There are many ways to do this in Photoshop, I usually put a black layer on top and put it on saturation mode.
This test should be made at the value laying, and throughout coloring up until the finished painting.