Tell-tale signs vary, if yours has gone you may have one or more of the following symptoms.

Creamy deposit in the cam cover and under the oil filler cap, caused by water getting into the oil.

Oil in the water.

Oil or water leaks from the engine block.

Gradual loss of coolant with no visible cause. (e.g. max to min in a week or so, if it occurs over several months this would not neccesarily indicate a problem)

Misfiring when cold for the first few seconds after starting up, caused by water getting into one or more of the cylinders.

Persistent misfires, especially when using heavy throttle at low revs caused by leakage from the cylinder past the head gasket.

Overheating (See other post on this topic)

Costs to repair can vary, budget on around 150quid for a garage or 60quid to do the job yourself, as the head is alloy and may need skimming in a machine shop

source: http://www.fiatforum.com/showthread.php?t=1841