Today Patricia T. O'Conner was on the Lopate Show to talk about language and grammar and to answer listener questions on the topic, and Natalie from Westchester called to shared a trick she uses to figure out when to use "who" and when to use "whom" in a sentence.
She explained: If you would answer the question with "he" or "she," you should ask the question with "who." And if you would answer with "him" or "her," you should ask the question with "whom."
Which means "Whom does this shirt belong to?" is correct because the answer would be "It belongs to him (or her)." You would ask "Who is going uptown on the A train?" because the answer is "She (or he) is going uptown on the A train."
Knowing the difference between who and whom confuses many people, and this is the simplest trick for figuring it out that I've ever come across. Thanks, Natalie from Westchester!