Whom = Whom

WHOM = HIM (A NEAT TRICK)

Substitute 'whom' with the word 'him'. If that part of the sentence still makes sense, then 'whom' is almost certainly correct.

Sarah gave the tickets to whom?
Sarah gave the tickets to him. (< sounds ok - "whom" is correct)

('Whom' and 'him' are never the subjects of verbs. They are said to be in the objective case. This is why the trick works.)

WHOM = THEM

In order to perform this trick for plurals, you will have to substitute 'who' with the word 'them'.

The witness saw four boys, one of whom had a limp, leave the shop.
...one of them had a limp... (< sounds ok - "whom" is correct)