Duplicate your artwork and expand all type and strokes to outlines, this makes colour selection easier. Duplicate again according to the number of colours you're using. Group each separate duplication. Double click to open the group and select all. Now subtract one colour from your selection by using the magic wand tool holding the Alt key (for subtract). Make your selection white. You now have separated the first colour. Repeat for the other colours and bring down the Magic Wand tolerance if your colours are too close for the magic wand to select it seperately.
I can now objectively look at what's happening in my composition taking it one colour at the time. What I am looking for is anything that's off, one area that's busier that the others, one colour that'stoo dominant, isolated bits and shapes that are too small. What I want is an overall composition that has a certain rhythm to it.
In this case the little ornaments on the wall in the chapel look too small and I think it will help the composition to make those objects a little bit bigger. I also don't like the negative shape on the bicycles, it is too thin a triangle to fit nicely with the rest of the composition so I will be touching that up as well.
Using this trick allows me to distance myself from the artwork I've been staring at for hours and look at it with fresh eyes. In the same way you can mirror or rotate your artwork to see where your composition could improve. Hope this helps! Let me know what you do to achieve the best composition in your illustrations.