First, ensure that oil is being fed to both sides of the cam by removing the tappet covers and observing the oil flung off the rocker arms; if there is inadequate oil supplied, damage to the cam and rockers will happen rapidly.
If the oil supply looks OK, and an exhaust leak is suspected, use a length of rubber tubing with one end held next to your ear, probing around the suspected leak area with the other end of the tube.
