This shell script lets you check who can login to your server using SSH keys

gistfile1.sh

#!/bin/sh
#
# check who can login to a user's account
#
check_user()
{
    local IFS=:
    while read user home; do
        test -e $home/.ssh/authorized_keys || continue
        cut -d' ' -f3 $home/.ssh/authorized_keys | sed -e "s/^/user $user: /"
    done
}

cut -d: -f1,6 /etc/passwd | check_user