get fingerprint of a ssh pubkey

$ ssh-keygen -lf /home/bob/.ssh/id_rsa.pub
2048 d6:59:bc:0b:18:ba:17:15:41:fc:d0:2a:60:f4:7e:e8 /etc/ssh/ssh_host_rsa_key.pub

Taken from: http://www.cs278.org/blog/2007/02/15/get-ssh-key-fingerprint/