How do I know if its the head or head gasket?

Don't understand question ... either one requires the same amount of work to get too. Of course a replacement head is far more expensive than a gasket repair. ( If it's a quad 4 engine ... I have one you can have) If the engine was overheated it sometimes distorts the surface between the engine and the head causing the gasket to fail thus requiring an engine "tear-down" ... quite expensive probably $1500 or more. If the head is distorted then it will have to go to a machine shop to be made flat

Tests must be run. Cooling system pressure tester, 15 min. remove sp. plugs. crank over. watch for wet hole(s). Compression test. Cylinder leak down. These will all work. or, pressure in the cyls with shop air and take rad cap off and wait. watch for air escaping through the coolant. There is the combustion gas test, add dye to tester, hold over the cap area above the liquid with eng. run at op temp. observe color change in the dye inside the tester. Look for coolant discoloration on spark plugs, o2 sensor gets fouled by coolant to tell you what side? on some engines. But the rate of the leak, the type of evidence, the severity, they can all indicate. A technician learns by experience. There is a judgement call factor. There are also many tools to buy to get more effective and more precise. Ask one for help. That's why they go to all that school for years and buy all those expensive tools. So you can try to fix your own with a armload of guesses.

What is this

What is this. And if bad what will it cause it to happen