How do I know if a drawing or illustration is any good? I could critique the character of the line. I could rationalise the colour palette – analogous, harmonious, complementary, and so on. I could critique the composition – is it dynamic? Does it adhere to the rule of thirds? Is the cropping suitable? What does the angle say?
The other way is to simply ask, is there life on the page? Because if life isn’t there, no amount of illustration or image theory will make it any better.