[Mo_Gearhead]

Remove the radiator cap ( do they have one - or just a over-flow bottle?)

Anyway - check it cold and hot, You are looking for excessive bubbling in the coolant. Culprit = Head gasket

But it could also be an intake-runner leak also. Any tell tale signs at exhaust pipe?

Remember - If it isn't leaking it - it's digesting it.

Edit: You could pull the spark plugs and look for 'washout' on a particular cylinder.