An easy way the tell if your oil pan is steel or aluminum is to touch it with a magnet. The magnet will stick to steel but not aluminum. Please make sure you are checking the engine oil pan, not the transmission oil pan!
With the 3.0L V6 it can be either steel or aluminum. The 24V DOHC V6 will have an aluminum oil pan and I believe the 24V engine was only available on the LX trim line (there are usually badges to denote the 24V duratec engine). The older style 3.0 (pushrod engine) has a steel pan
You can verify by looking under your car with a flashlight. The steel pan will be smooth (it is stamped) and often is painted black. The aluminum pan will look like a casting I believe. I am referring the engine oil pan