When a student would present me with a photograph I would often look at it upside down. I don't this works for every image, but it is a good guide to see if some form of aesthetic composition is at play.
It's a sort of interesting thing that in painting/drawing one trick used is to look at a work in progress in a mirror. It's for a sort of related but maybe different reason, basically to see if you've not got something catastrophically wrong. It's surprisingly easy for your brain to just decide something you've started at for hours looks right. Looking in a mirror causes a swift and thorough re-evaluation.

