I don't know about you guys but I can tell when a ferrule is getting loose. The fly rod just doesn't feel "right". It loses what I would call "crispness" and takes more effort to cast.
Also you may notice that the guides are slightly rotated off axis from each other. Most folks will flip the rod over and sight down the guide side of the fly rod. I sight down the rod with the guides on the bottom. You should see an equal amount of guide projecting on each side of the blank. It seems easier for me to notice very slight rotation of one rod section from another using this method. Since you hold the rod this way anyway, you can do it quickly.
Basically though, whenever I feel that change in the castability of the fly rod, I reseat the every ferrule.
"Discovery consists of seeing what everybody has seen and thinking what nobody has thought"..........Szent-Gyorgy