In a bitcoin multi-sig wallet, can i determine which of the m of n keys were used to send bitcoin? I'm particularly interested in this for large n multi-sig wallets. For example, if m-of-n is 4-of-6, and 6 different people (e.g. 6 non-profit co-directors) hold each of the n keys. If one day all of the wallet is emptied just using 4 (m) keys, will the 2 (n-m) people be able to determine which were the 4 keys and thereby the 4 co-directors that emptied the wallet?
The signatures and public keys have to be supplied to spend the coins so yes you can tell by looking at the spending transaction on the blockchain.