On the suspect rocker (if visible to the naked eye) use a dial gauge to measure lift of the lobe. If they all sorta move measure each one individually.
If it is a worn exhaust lobe you may hear a chuffing (like air compressor noise) through the air cleaner as the gasses cant all get out the exhaust port and try and backfeed.
I have seen a lobe on a Mack truck completely gone, nothing but round. It can occur when the hard facing on the cam fails, the rest of the softer metal then goes rather rapidly.
An
oil sample analysis would show this also.