Know your stuff
Definition 1 of 1
Pro Tip 1/3Professional Expertise
Use 'know your stuff' to show high-level knowledge or skill in a job or hobby. 
He's the go-to guy for software issues; he knows his stuff.
Pro Tip 2/3Colloquial Use
'Know your stuff' is conversational and might not suit formal writing or speeches. 
In a casual chat about cooking, you might say 'She really knows her stuff.'
Pro Tip 3/3Positive Praise
Saying someone 'knows their stuff' is a way to praise or recommend them. 
Ask Tom about your car's engine, he knows his stuff.