the o2 sensor that has the most control over fuel strategy is in front of the cat, and most are heated. Look at the sensor itself, it will be either a one or four wire sensor. The four wire being the heated one. EGR systems are designed to work with backpressure, hence the back pressure transducer that feeds vacuum to the egr.
If you have a later model EFI truck with the "diagnostic" box, you know the one you jump to check codes, you can probe the box to read o2 sensor voltage. Just use a Digital voltmeter, probe from the o2 pin to ground(e1 pin I think). Lean is low voltage, say .1 to .2 volts, and rich is .7 to .8 volts. A properly working sensor will toggle back and forth from rich to lean. Just make sure the truck is at operating temperature.
