How can you verify the change in the default user shell for 'joker'?

You can verify the change in the default user shell for 'joker' by using the following command:

sudo grep -w 'joker' /etc/passwd

This command searches for the entry corresponding to the user 'joker' in the /etc/passwd file. If the change was successful, you should see /bin/bash at the end of joker's entry, indicating that bash is now the default shell for the user.