how do i know which key has mined bitcoin?
You have to load the keys into software which can scan the blockchain and search for transactions that belong to them. The simplest way to do this is to just start Bitcoin Core and let it sync. During the syncing process, Bitcoin Core will also be scanning the blockchain for any transactions that belong to any wallets that are loaded, so once it is synced, you will see any transactions (and thus Bitcoin) that belong to you.