Engine head is bolted to cylinder with a gasket to seal the area. If gasket or head is not okay then it will cause leaks and coolant may enter ur engine oil.
Symtoms: Weak engine/no power/missing and back firing, White smoke, oil and bubbles in radiator, overheating, low compression.
However there can b other reasons of all symptoms mentioned above.
U cannot judge that engine head is weak until u inspect it physically - might be u r facing issue coz of blown gasket.
A bad/weak engine head is basically a deshapped metal which cannot retain it shape. If engine extremely overheated then engine head may have cracks.
Normally we do facing of engine heads to bring it back in shape but it is not possible every time.
Hope u r clear now.