As far as I am aware MarisaWasHere is right, in that 'who' is the subject of the pronoun and 'whom' is the object of the pronoun.
The verb 'to be' doesn't take an accusative object therefore 'who' should be correct. But then the 'determining' and 'turn out' part threw me, because 'to determine' something, takes an object... does't it...?