Unlike a lot of Border Collies, Casey will usually let you know when he's red-lined after about 10 or 15 minutes of non-stop activity, in this case Frisbeeing. One big plop, followed by a huge smile that says, "Wasn't that something? Am I special or what?" is the give-away to wrap it up.
Or is it, "Thanks, man. We gotta do that again...real soon"?