It is very simple. The head has to be removed.
Once off, the bad valve, or the bad cylinder wall/piston will be revealed. If the valve lash is present in any amount, the head comes off.
It may be a bad valve, or at worst a broken ring and a scored cylinder wall. In any event, run the piston to the bottom and carefully inspect the cylinder walls for a vertical scratch or two. If present, the upper ring is probably broken, and if it has been for some time, the piston will be damaged in the groove.
Driving it will further damage the piston if it is bad, not the valve. But the exhaust emissions will be so high that you will be way past a "gross polluter" and on that point alone, one should not drive an engine that is hurting so much.