If you pull two cylinders on one side you can use a flashlight and inspection mirror or a borescope to view the cam and all the lifters. Failure will usually begin with the lifter face, it will get slight pitting, then progress to all out lifter face breakdown and spalling. Then the cam will get rough and destruct.
We did this inspection on our airplane at prebuy and it looked great. IO-360A3B6D. Then 250 hours later we had 3 spalled lifters and a trashed engine. You can't see the small pits at the center of the lifter face, which we must have had.
We recently tore down a Continental E-185 which looked good. However, 8 of the 12 lifters had early signs of slight pitting. Its very small, but if the lifter faces dont look like chrome and zero scratching or pitting, its going to have to be split and the cam and lifters replaced.