Duck test - If it looks like a duck

"If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck."

Overview

Common idiom: The precise origin is unclear, but the phrase emerged in the mid-20th century and is widely attributed (without definitive evidence) to James Whitcomb Riley (1849–1916), an American poet, who allegedly said:

"When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck."

However, no confirmed source from Riley himself has been found.