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know a hawk from a handsaw
To be able to tell two things apart. The phrase comes from Shakespeare's Hamlet. Of course I know which twin is which, I know a hawk from a handsaw, after all!
know chalk from cheese
To be able to tell two things apart (especially by recognizing their differences). Of course I know which twin is which, I know chalk from cheese, after all! Leah has a beauty mark under her left eye, and Deena doesn't.
know for a fact
To have definitive knowledge of something. I know for a fact that he cheats in cards! I saw the ace up his sleeve. My mom says she knows for a fact that I'm getting the internship at her company, but I won't believe it until the recruiter calls me.
know (something) inside and out
To be well-versed in and/or very familiar with something. I'm ready for this exam—I know genetics inside and out! I think Mary will do great at the audition, now that she knows the dance inside and out. These cabbies know the city streets inside and out.
know (one's) beans
To be knowledgable about and/or very familiar with something. I had no idea you were such a good piano player—you really know your beans!
To have a familiarity with someone; to know who someone is. Such familiarity can range from close friendship to having only met someone once. Oh yeah, I know Mr. Meyers, he used to be my neighbor. He knew Vanessa long before she became a famous singer.
See also: know
know what's o'clock
To be attentive and aware of all that is going on. Those students thought they could get away with cheating in my class, huh? Too bad I know what's o'clock!
See also: know
know which way the wind blows
To be able to anticipate how a certain course or situation is likely to develop. These politicians know which way the wind blows regarding public opinion on the issue, and you can bet they'll vote accordingly.