try this command:

sudo cat /var/log/authlog | grep Invalid

if you've had your server open for any length of time, a large number of attempted logins will scroll by. Here's how to protect against these attacks ever succeeding:

groupadd ssh
vi /etc/group (and add yourself to the ssh group)

vi /etc/ssh/sshd_config
AllowGroups: ssh
PasswordAuthentication no

now add your public key your ~/.ssh/authorized_keys file

restart the ssh server:

sudo su -
(hit return to accept the default terminal selection)
pkill sshd && /usr/sbin/sshd