Perhaps you were thinking of one of these similar phrases:
* "Know your onions": This idiom means to be knowledgeable or skilled in something.
* "Know your stuff": Similar to "know your onions," this means to be well-versed in a subject.
* "Know the ropes": This idiom refers to being familiar with the details and processes of something, often a specific job or activity.
If you can provide more context about where you encountered the phrase "know hair," I might be able to offer a more accurate interpretation.