I'm sure the club has pretty good data (more than all of us anyway) on physical decline. It's easy to judge using metrics like
acceleration (link to Arsenal stats), sprints per game (offensive sprints vs defensive sprints), distance covered etc. I'm curious though what this data looks like for some of the more recent flops like Torres, Sanchez and Mata and if it actually showed a precipitous decline in physical stats before the player was sold.
The metal side of things are harder to judge. Does he have the same hunger and desire to win? Or is he done and looking for one last pay check? I think Ole would have to talk to the player, pull some strings and understand where the player is.