SUBJECT? WHO.
- Who left this unicorn in my garden this morning?
- Who is responsible for letting the bats into my belfry?
- This tatty old sock belongs to whom?
- You need my spyglass to attend the opera with whom?
Some of us had, at some point in our academic careers, teachers who taught us to swap out the words who/whom with the words he/him (or she/her, if you will) within the sentence to find the appropriate usage. Personally, I feel that's still way more complicated than necessary and will lead to further confusion in the form of nonsensical sentences. If one needs a pencil and paper to practice one's queries upon before putting them to one's interrogee (okay, yes, I made that word up--sue me), then one will have wasted precious investigative time and energy.
Allow me to simplify. Look instead for the answer to the question. If the answer will be him/her, use whom; if it will be he/she, use who. Who? He. Whom? Him. For example:
- Who/m is going to cook the holiday turducken this year?
Let's look at another example of the opposite inclination:
- Who/m would you like to receive your complimentary hand-knitted cat hair yarn purse?
...and yes, such a thing really does exist, as do unicorns, bats, and turduckens.