English Grammar Made Easy: How To Use Who and Whom

http://www.public.asu.edu/~gelderen/314text/images/whom.gif

the following explanation is taken from here

The Rule.
Use the he/him method to decide which word is correct. he = who, him = whom

Examples:
Who/Whom wrote the letter?

He wrote the letter. Therefore, who is correct.

For who/whom should I vote?Should I vote for him? Therefore, whom is correct.

We all know who/whom pulled that prank.

This sentence contains two clauses: We all know and who/whom pulled that prank.

We are interested in the second clause because it contains the who/whom. He pulled that prank. Therefore, who is correct.

(Are you starting to sound like a hooting owl yet?) hahahaaa....

We want to know on who/whom the prank was pulled.

This sentence contains two clauses: We want to know and the prank was pulled on who/whom. Again, we are interested in the second clause because it contains the who/whom. The prank was pulled on him.

Therefore, whom is correct.