Fluctuating neg. to ground on ignition coil

Stephan D said
Wouldn't that show up in the compression test or create vibrations at other RPMs than idle as well?

If not, how do I check if a cam lobe is wiped out, removing the valve cover to see if all rocker arms move in a similar way?

You could have compression with a bad cam.
Visual inspection of rocker arm movement is the easy method.
Kill the ignition and rotate the engine using the starter.