So, I think I've (maybe) found away to check a drawing before going to a mirror.
I'm not sure how I came to the conclusion, but a while ago I got the idea that the reason why we miss mistakes in our drawings is because we complete the drawings in our mind before we actually finish them, as we're still drawing them. We get a mental picture in our head of what we're making, and it conflicts with what we see on the paper. This mental image can stick with us for a long time, even after the drawing is finished; there's no true end to something you paint/draw (you could technically work on it a lot more), so maybe its always 'unfinished' in a person's mind. After a while what you imagined slips from your memory, and you see your image more clearly.
To try and fix this, as I draw, I look at a drawing as if it is already complete (and so what I imaging is somehow overridden), regardless of what stage it is at. I figure that this helps with my ability to more accurately judge a drawing I make. Thoughts?