Some say that ‘whom’ is going out of fashion and that you should not bother about it at all. This sounds too easy don’t you think? So, in the meantime …
If you could replace the word in question with he/she/they/I - it’s who.
If you’d say him/her/them/me - it’s whom.
It seems to be working for me but I’m sure that like all good rules there will be an exception that will cause me grief.