know what (one) is talking about
To be adequately knowledgeable about or experienced in something one is discussing. Mister, I don't think you know what you're talking about. There's no way these old pipes can withstand that kind of pressure!
know what you’re ˈtalking about
(informal) have good knowledge of something; be an expert on something: I really enjoyed that lecture. She certainly knows what she’s talking about.That history teacher doesn’t know what he’s talking about. He makes a lot of mistakes.