Quick Tip: Think about "he or him" and listen for the 'm' sound.
If it’s a question, answer it with he or him. Did you see who/whom picked up my notebook? He picked it up. ➜ Did you see who picked up my notebook?
If it’s not a question, ignore the first half and substitutehe or him in the sentence. I'm not sure who/whom I'm going to vote for. I'm going to vote for him. ➜ I'm not sure whom I'm going to vote for.