I would say their sexual appearance in particular. So far as a I know, all mammals and many other species as well, will check out anyone that seems to have potential as a mate.it would seem that 'checking someone out' refers to their physical appearance.
Judging someone by their physical appearance indicates that you may only be interested in them physically...not emotionally or mentally.
Why is that wrong? Because a person is more then the sum of their parts.
So? Some one walking down the street with a nice A$S can be enjoyed for the quality of that A$S without any sort of detriment to their mental or emotion qualities. If I like how food looks does that mean I like how it tastes or smells less?