when the sparkplugs are out of the heads , you also can turn loose the valve rockers till the springs are decompresed, use a pressure airgun into the sparkplug holes , when there is one leaking than there is the problem.
this way it is not needed for turning the engine for protect further damage.
maybe you have luck and find the problem with a flexible magnet.
but most of the time there is a valve bented and one head needs pulled from the engine.
good luck,
Simon