When to call a girl on her lies..?

Lets face it girls lie.. they just do. They rather tell us what they think we want to hear. And sometimes there right, but thats another thread.. I am pretty good at telling when a girl lies to me, but i usually don't call them out... because to press it would make me look like i care too much about trivial ****..and thats not cool. So when IS an appropriate time to call her out, and whats the best way to approach it.