I went to change my oil today and found brown mayo stuff on the inside of the fill cap, and also was low on coolant. How can I be certain it's the head or the intake gasket? I hate to do all that work just to find out it's the other one that needs replacing.

99% positive its the intake.
but for 100% accuracey - take to a shop and atleast pay for the pressure test and they will tell you exactley where the leak is. and it sounds as if you are handy, you can then do the repairs yourself.

I second bleepster.

I had the same stuff on both 3.4 engines I own and replaced both. It was real PITA but it is over now and hope this thing lasts until I can get more reliable set of wheels.

Good luck man. BTW if your vehicle has less than 100,000km on it dealer will generally replace them for free if you approach them correctely.