Dan, a couple different answers for you:
1. Marketo does NOT automatically unsubscribe a Forward to Friend recipient if they already exist in your database. So no need to worry there. (I had wondered this earlier myself, and did some testing to find out)
2. For the filters, you might try looking for just the field "Source". That could be the label for the Lead Source field in Marketo (depending on how your field mapping is set up).
And "Sent Forward a Friend Email" is also its own filter and trigger that you can use. See my screenshot below for these examples. (However, they listen for when a current lead SENDS a FtaF email, so you will have to click through into the Activity History to see the recipient.)
You may not even need to build the campaign you explained, since existing leads won't be unsubscribed. But if you still wanted to search for them just in case, you should be able to with filters and triggers shown below.