I haven't unsubscribed from a list in years, perhaps decades, despite being subscribed to a few. So I can only tell you from memory. In Thunderbird, I believe I've see a "List Unsubscribe" button in the list of actions available, alongside "Reply-All", "Edit as New" and so on.
In GMail I believe senders that have this implemented now have a big blue UNSUBSCRIBE button next to their email address at the top of the message.
In GMail I believe senders that have this implemented now have a big blue UNSUBSCRIBE button next to their email address at the top of the message.
Neither appear if the headers aren't there.