know what you're doing/about
know what (one) is about
To be adequately prepared or knowledgeable enough to do something; to be confident in one's abilities. You couldn't find a better leader: he knows what he's about, he stands up for his values, and he isn't afraid to put the wellbeing of other ahead of his reputation. I know her strategy may seem unorthodox, but trust me—she knows what she's about.