SSH login attempts are logged in
/var/log/system.log. Grep for
sshd in that file and
you'll get the logins.
Here's an example of a failed login followed by a succesful login:
% grep sshd /var/log/system.log
Nov 10 22:30:22 Lanfear.local sshd: error: PAM: authentication error for teun from localhost via 127.0.0.1
Nov 10 22:30:36 Lanfear.local sshd: Accepted keyboard-interactive/pam for teun from 127.0.0.1 port 51239 ssh2
Nov 10 22:30:36 Lanfear.local sshd: teun [priv]: USER_PROCESS: 98453 ttys004
If you're worried about brute force attempts on your password the best thing to do is disable password authentication and only use key based authentication. You can disable password based authentication by editing
ChallengeResponseAuthentication yes to
ChallengeResponseAuthentication no. Make sure your SSH keys are working before you do this or you won't be able to login remotely.
There's no fixed rule on when a password is weak or strong, but in general when using passwords I'd use a passphrase consisting of multiple words and some numbers and/or puntuation.