know what (you) are talking about

to speak in an informed way because of your experience or education.

He doesn't know what he's talking about. He's never even been to Alaska.

• get goat
• get going
• get grief
• get groove on
• get hands dirty
• get hands on
• get hands on someone
• get hands on something
• get head around
• get head around something
• get head down
• get hell
• get her claws into
• get her claws into someone
• get hold of
• get hold of someone
• get hold of something
• get hold of the wrong end of the stick
• get hooks into
• get hooks into someone
• get hooks into something
• get hopes up
• get in a lather
• get in a rut
• get in gear