To look at someone's features in order to determine that person's level of sexual attractiveness and the extent to which you would like to chuck it in her/have him chuck it in you.
After I nailed her little sister and didn't call back, Kim went on a major bitching tirade. I didn't mind though, since she was wearing a low-cut shirt and it gave me a good chance to check out her amazing rack.
to check someone out, is to look them up and down and assess how atracted to them you are, based on physical apearence only. Can be done by both males and females. If your a straight guy, you would "check out" girls but not other guys - even if you apreciate that they are atractive, this is different to being atracted to them. Obviously if your bi or gay this would change according.