You can't know. The woman will know it's right when you take initiative without knowing it's safe - sometimes. If you knew in advance it wouldn't mean much to her that you made a move - any guy can do that. And it wouldn't be the first move, would it?
That being said, this does raise prickly issues. It is becoming more and more unacceptable for men to make unwelcome advances. So the risks are indeed going up. Could they get to a point where men won't want to make advances? Who knows, but probably the pendulum will swing back. And I understand the younger generations of women are MUCH more aggressive.
Anyway, I'm not speaking much from experience. I'm fairly shy and have generally waited for very clear signs from a woman. I wait a LONG, LONG time, but when the time comes there is no sweat about it all; it just happens.
In short, there is no trait women like more than confidence. However, note that women probably observe this in how you conduct yourself in general and are not necessarily as concerned in precisely how and when you make a move.
Probably the less you think about it, the better off you'll be.
The author referenced below is pretty good. Note, however, that, if some technique were to become to widely known, it would stop working.
Also, women do not all fall into the same patterns.
Hope that helps.