One thing that just came to mind, that I thought I would pass along, is how we are actually training our eyes and minds to detect minute details in shading and features.
Here are a few things I do to help me try to notice the differences between my drawing and the reference photo.
Sometimes I rotate the drawing sideways and upside down. It helps me see things I don't normally see?!? Especially when drawing eyes, and when I'm trying to determine exactly where the eyebrow borders are. I don't need to tell you how a few hairs in the eyebrows can make all the difference in the world?
I sometimes tape it to my desk too, because the edges of the paper curl up, and it effects my pencil strokes?
Holding it in front of a mirror helps too.
But the MOST effect way I detect differences between my drawing and the reference photo, is when I think I'm finished, I take a photo of my drawing (before I spray seal it) and then I view it on my computer, side-by-side in THUMBNAIL version, next to the reference photo. For some reason, seeing them in thumbnail format helps because they are both smaller versions of what you've been staring at for hours.
Better yet, I view them back and forth very fast using "Windows Picture and Fax viewer". You will be amazed at all the differences you can now "SEE" that you could not see before?
Hope that helps?