It is not technically possible to know who forwarded an email from a mail client so this is not supported. If a mail is forwarded to someone it is as if any open/link click is performed by the original recipient of the email.
You can set up smart lists in your instance to see the leads who sent or received an email using the "forward to friend" link. This would not work for traditional email forwarding.
You can search for "forward" in the list of triggers and filters for:
-"received forward to friend email"
-"sent forward to friend email"
-"not received forward to friend email"
-"not sent forward to friend email"
This Community article may be helpful to you if you are more visual (scroll to bottom).
Where can I find this Community Article about setting up a smart list to track "Forwarded to a Friend" that was posted in the response? When I click on the link it just takes me to a search bar.
If you run a search there are a few topics on this subject. Below are resources, but just search for "forward" in your smart list filters.
When sending emails to external domains, the only way to protect content is to encrypt and password protect any files being sent and/or direct recipients to a gated web portal where your content can be viewed following login.
Also, please note that Jamie's example (clicking a forward to friend link within the email itself) will not mean that anything is tracked when the user clicks the "Forward" button on their email client, which is probably the most common way emails are forwarded.
Thank you both! Much appreciated!