As a rule of thumb, imagine your phrase saying "him" or "he". If it makes sense to say "he" then say "who, otherwise say "Whom" Examples: Who is guilty? - He is guilty - Accurate Please send it to him - Sorry, send it to whom? - Accurate. Let him know. Let who know? - Inaccurate

This should explain it quite well: http://web.ku.edu/~edit/whom.html