Is there an art in understanding a woman's feelings?

Okay so, this has been a question that has been in my mind for so long as I can think of really.

To the men, what's a way to be able to easily tell how a woman is feeling at anytime when you are talking to her?

To the women, what's something that men are almost always missing when it comes to signals you may be trying to give out as you are talking with them?
Is there an art in understanding a woman's feelings?
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