you are correct. With the engine off, remove the air filter and take a look down through the throttle body. If you see any pooled oil, you have a blown plenum gasket.
my symptoms were rough idle, low power, bad mileage(9mpg), oil consumption(1 quart per tank of gas). I did not have a noticeable amount of smoke out the tail pipe.
I took some pics of my plenum repair and posed them here:http://www.ramforumz.com/showthread.php?t=127559