Worn linkages are easily to diagnose, remove the airbox and feel around the cups for movement. On the larger cups the rubber inserts are only held on by two small sections opposite each other, over time they deteriorate and make the gearchange feel sloppy before eventually popping off. If theyre in good nick you shouldnt be able to move them off their pins atall, only around them.
If you cant feel any unwanted movement in them, it could the be selector inside the box itself, theyre a common fault.