Sometimes you're doomed before you've even make your approach. We don't want you to waste your time (or take an unnecessary blow to your ego), so please, take a minute to assess the situation before flinging your body into the fire.
Does she appear to be talking emphatically with a friend, possibly giving her advice or engaged in serious conversation? Expect to approach her and receive an irritated "shoo" look in response. Does she have one eye half-open and her heels removed? If she's had her way with Jose Cuervo, she may not be the best candidate, unless tequila-scented projectile vomit is your thing (please, don't answer that).
And most importantly, if she's lingering by a particular dude, unless you're looking to tint your tan with black-and-blue accents, maybe play it safe and seek elsewhere. A quick game of Clue at the bar can save you serious stress on the battlefield. Trust.