judging - What parameters can I use to evaluate a "Perceptual Image Quality"?

Might be a naive question, but I want to fix few subjective metrics / parameters which would let me decide image quality.

I mean, I can use some objective metrics like Mean Squared Error or Mean Absolute Difference, if I have the "golden reference image", but in real life situations, most of time there is no reference image. In those cases, how do I decide the quality of image by viewing it?