I found this post at writerbynature.com and I thought this was a good way to determine which to use -- who or whom?
This is one of those tricky rules that if you get it wrong, it makes you sound stuffy or pompous, but every once in a while you run across a place where you need to use the word whom, so I found this really helpful:
Who/Whom falls into my pesky words category.
I spent years reaching for my style manuals whenever confronted with these words. Until I learned a couple of neat tricks.
In the past, I might have told you to use who when you mean the person taking action and whom when you mean the person is having something done to them.
But there’s an even easier way to deal with who and whom:
Try substituting him or he (her/she) for who or whom.
For example: Who/Whom do you admire? Test: Do you admire him? Do you admire he?
Do you admire him? Is correct and as luck would have it, him and whom both end with the letter m.
Who/Whom broke the vase? Test: Him broke the vase. He broke the vase.
The answer: He broke the vase.
Try practicing with these sentences:
Who/Whom stepped in the mud?
Who/Whom do you trust?
Who/Whom is going to the concert?
To who/whom should I address this letter?
Use this test when in doubt and you will be able to see whether who or whom is the right word. (~ jj_murphy The Writing Life)