I was going to write a flow chart to illustrate how to decide if something is sexist, but then I realised I only have one criterion: would this be offensive if the genders were reversed? For instance, middle-aged women drooling over shirtless teenage boys in Twilight. Reverse the genders and you suddenly feel very uncomfortable.
Mokalus of Borg
PS - I don't go in for double standards, just in case you're wondering.
PPS - You might be surprised how much of it you see, when you look.