Get a list of private key paths known by the SSH Agent connected to the SSH Auth Socket

ssh_agent_keys.sh

#!/bin/bash
function binary_dump_of_keys(){
    echo -en '\x00\x00\x00\x01\x0b' | nc -q 1 -U $SSH_AUTH_SOCK
}

function filter(){
    strings | sed '/.ssh/!d; s/^[^/]*//'
}

binary_dump_of_keys | filter

# example output:
# /Users/bbronosky/.ssh/id_rsa.pem
# /Users/bbronosky/.ssh/root_bootstrap.pem
# /Users/bbronosky/.ssh/aws_master.pem

I leaned heavily on https://ptspts.blogspot.com/2010/06/how-to-use-ssh-agent-programmatically.html for this.