You can't always look at a head gasket and tell it's bad but look for carbon buildup or stains around the mating surfaces and on the gasket itself. If you see anything that looks like a little 'flair out' it might be a sign of a breech. While you are at it, check the spark plugs and see if any have calcium deposits or are unusually clean compared to the others. If water is getting in there, it will clean the carbon off of the plug and can help make it easier to spot which cylinder is leaking.
It could also be a cracked head too.
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