You should boot on the Recovery HD and use Terminal there to unlock the drive using the recovery key (you can't check the recovery key if you are booted on the crypted disk, since the unlocking process will fail: already unlocked).
To start with, you will need to identify the Logical Volume UUID of the encrypted drive using the diskutil corestorage list command:
diskutil corestorage list
Running that command will give you a listing of all Core Storage volumes.
Once you have the UUID of the encrypted volume, you can then unlock the encrypted volume using the following commands:
diskutil corestorage unlockVolume UUID
When prompted enter the passphrase you would like to test.
This article provides more information and screenshots, Unlock or decrypt your FileVault 2-encrypted boot drive from the command line.