Ford Repair: 4.6 liter ford motor problems, blown head gasket, head gaskets

Question
Having a problem with spark plugs going bad in cyclinders 3,4.  People are telling me that water is cracking the plugs.  Truck is running very rough, how do I determine for sure if the head gaskets are bad or if I have a cracked head?  Before the 4 cyclinder problem, I could change out the plug every few months and the truck would run fine, but now I've changed them out again and nothing is happening, the truck is running rough.  The cel codes have only said that it is a misfire on those cyclinders.

Answer
A compression test will sometimes show a blown head gasket.If the plugs are cracking, then you're head gasket is blown.Change it before it warps the head.After removing the head have them checked at the machine shop.Another alternative would be to find a salvage warranty.You could pick up a used engine with reasonably low mileage at either Advance or AutoZone.I know that Advance's used engines come with a 1 year warranty.That says a lot to me.I got a transmission from them that looked like it had just been rebuilt for my 1994 Thunderbird for $250.