How can I test access to encrypted (FileVault2) Time Machine partition?
You can confirm this by booting from the Recovery Partition on the Time Machine disk and choosing to restore from a backup (without proceeding with the restore).

Here are the steps:


Connect the external hard disk with the encrypted Time Machine backup
Boot (or reboot) the system and during startup hold the Option key so that it shows you the list of available volumes to boot from
Select the Recovery HD from the Time Machine disk to boot from it
Choose Restore From Time Machine Backup from the list of actions
Select the encrypted Time Machine volume from the list
Enter the Fila Vault 2 password when prompted
Wait for it to show the list of available backups from the Time Machine volume
Restart the system now that you have verified the availability of the backup