I know a hawk from a handsaw. Handsaw is a corruption of hernshaw (a heron). I know a hawk from a heron, the bird of prey from the game flown at. The proverb means, I know one thing from another. (See Hamlet, ii. 2.)

Source: Bartelby.com – E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.

“And I am a Wild Card, Mark, and I do know a hawk from a hernshaw.”

“What the heck is a hernshaw, anyway, man?”

“Nobody knows. Don’t let the junior-college Hamlet commentators tell you the word is really ‘handsaw,’ though. That’s hooey.”

Source: Turn of the Cards, Victor Milán.