[Originally posted in answer to a question on Quora on January 15, 2016…]
Short answer: When it doesn’t bother her.
Longer version: Unless you have enough rapport with a girl to know what she’s thinking (You don’t. Period.), it’s safe to say that unless she’s actually brought up the subject (“Does this belt make me look fat?” “Can you see my striped panties through these slacks?”), it wouldn’t be polite to get caught gawking.
That said, there are some non-verbal cues that may help in this regard. Some rules I’ve lived by include:
1) If a woman has a tattoo, she is giving everyone permission to look at that tattoo at any time — wherever it happens to be. But only there.
2) If a woman wears a garment that is purposefully cut to reveal more of her than what everyone else in the environment (office, school, market) is wearing, she is giving you permission to look there. But only there.
3) If a woman has engaged you in conversation, the thing to look at is her face — and only her face — unless the words coming out of her mouth direct you to something else.
4) If a girl is dancing, she is giving you permission to look. But close your mouth.