Head Gasket or the Upper or lower intake manifold gasket ??
 

I have a 2002 Chevy Impala 3.4L V-6 and there is oil leaking into my coolant. There is currently a lawsuit against GM for the Antifreeze they use (Dexcool) which eats away the nylon/silicone gaskets they use. My question is how do you know if it is the Head Gasket or the Upper or lower intake manifold gasket ??