Since I have no knowledge of the following, I had to consult an outside source and according to an actual female, my friend Zander, girls try on at least 12 outfits before they go out. I think that is subtle enough. From what I hear, it is appropriate just to nod one's head and consistently say that that looks great. Venturing in another direction, for those of you who don't have the pleasure of taking communal showers like myself, let me just tell you, you're missing out. The following rules apply for all locker room situations and usage of urinals.
Eyes are to be kept above the midline and small talk kept to a minimum.
But since it is only human curiosity to take a peek, fine, but any type of ogling, staring, or daydreaming is completely unacceptable. Let's just assume I'm fine and until you grow some breasts and start using the other bathroom, I don't want to chit-chat. If my elbows aren't on the table, I have very few options with what I can do with my hands. Imagine looking across the table from me, and you don't see my elbows. It can only mean a few things.