There's the ever faithful xeroxing, or you could turn your back to the mirror and look over your shoulder to investigate. Unless you have a hand mirror, or two floor length mirrors that you can position to look at. (I have not put a lot of thought into this, I swear, but I have had to make sure that my back looks as good as my front.)
It -- depends. Does it lead to lengthy worship of said ass? Because I could possibly stand it then.
I can be superficial, but by the same token, mostly no. Are a lover's words as pretty when they've said them, or are they still made more beautiful when you know they'll follow them up with actions? I want pretty and substance. Call me greedy.