For Spy questions you always want to cut the list in half.
As such, the most direct solution to me is to write down the amount of posts of each *living* player, checking the middle of the list and asking "Does the killer have more than X posts?" Only problem is if two people near the middle have very similar post counts such that their positions could change between the asking of the question and the response of the GM
As such, the most direct solution to me is to write down the amount of posts of each *living* player, checking the middle of the list and asking "Does the killer have more than X posts?" Only problem is if two people near the middle have very similar post counts such that their positions could change between the asking of the question and the response of the GM