Microsoft Exchange: Mailboxes forwarding to Specific Mailbox

 The following will list all mailboxes on your tenant that are forwarding to the specified recipient.

$RecipientCN = (get-recipient johndoe).Identity
Get-Mailbox -ResultSize Unlimited -Filter {ForwardingAddress -ne $null} | Where-Object {$_.ForwardingAddress -eq $RecipientCN}

Huge thanks to Grant for answering pr0digy's question over on Stack Overflow