How do I go about correctly diagnosing a possible head gasket problem?

2000 F150 6 cylinder 95,000. Truck has been running great but I saw a drop in power last week and gas mileage dropped from 18 mpg to 12 mpg. While driving the truck the windshield fogged up inside with oily substance. Checked dip stick and oil level looks ok, but possibly a greenish tint to it, I'm realy not sure. Looked in the radiator overflow and can't tell weither antifreeze is contaminated with oil or just dirty. Oil fill cap and down into the tube covered with white/yellow foam. When I get home this morning I'm going to change the oil, and then go from there.

Any help would be much appreciated.

 

well the white stuff under the fill cap is normal, the oiling residue could be a leaking heater core. Unless you are putting white smoke out the exhaust, or antifreez in the oil, your headgaskets are fine