Here's a work-in-progress, from a series of city scenes. I can see the negative shapes of the sky better when it's turned upside down as well as how the composition is working.
My favorite method, though, of checking on how I'm doing is taking photos and looking at them on my computer screen. I suddenly see mistakes in drawing or in value a lot better by looking at a photo. Some artists use a mirror to do the same thing. I don't really stand on my head (I never could do that!). Do you use these methods to assess your progress?