Ya know we addressed this on a 1,000+ thread and couldn't agree!
You could turn that around and ask how does a woman know?
Its a matter of personal reference, I like a man that see whats he wants and goes after it.
Also men need to pay more attention to a woman's body lanuage, its defintely a clue to the old question: to approach or not to approach
However I say when in doub, go for it!
That takes alot of balls, especially for someone who spent the last decade or two supressing that very instinct. Like GWB says, it's hard work (absolutely no pun intended).