If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.
Meaning: "It is usually safe to identify somebody as a particular type of person when his or her appearance, behavior, and words all point to the same conclusion."
Source for meaning: Martin H. Manser (2007). The Facts on File Dictionary of Proverbs. Infobase Publishing.
Reportedly coined by James Whitcombe Riley, sometime before his demise 1916. He wrote: When I see a bird that walks like a duck and quacks like a duck, I call that bird a duck.
Made famous by the then Governor Ronald Reagan's use of the expression 1967, in an interview with a journalist. (Cryer 2011, p. 163)