On fedora 20+ the login attempts and successes are saved in /var/log/audit/audit.log . This log saves the login attempts (failures and successes), and the key fingerprint used for login attempt is saved in the field named fp.

You can compare the logged in key fingerprint with the fingerprints in the authorized_keys by running it line by line through ssh-keygen -l

A detailed explanation with respect to ssh logins and their security and intrusion detection is here: http://vpathak.tumblr.com/post/121343814158/fedora-audit-log-with-love-from-russia