when I want to sent btc, how do I know which from all of mine keys is used?
That depends on what wallet you are using. Some wallets let you choose, others don't. In the case of Bitcoin-Qt, you don't get to choose which keys are used, and the wallet doesn't tell you which keys are used. It hides all those technical details from you. It will give you the transactionID which you can look up somewhere like blockchain.info if you want to see what keys were used.