Here is how to tell. Take all your glow plugs out, and drain your oil. Presurize your cooling system, and let it sit. One of two things will happen. You will get coolant running out of the bottom, or flooding out of a glow plug hole. If it comes out of a glow plug hole, your in better shape, then if it runs out of the bottom. Unless you have a cracked piston, water out of the bottom will almost always be cavitation. If it comes out of a glow plug hole, you will know what cylinder has the problem. Bring the piston in that bore to top dead center on the compression stroke. Let it sit again. If more water comes out the glow plug hole, then it is either the head gasket or a cracked head. If it comes out of the bottom, then your back to caviation. Here's hoping your top is wet and your bottom is dry!