It's usually located near the distributor area, if the engine has one, or near where the distributor *would* be if it were distributor-equipped. Look for a sensor with a single wire connector, often mounted near the camshaft gear or the timing chain cover. It's usually a smaller sensor than the crankshaft position sensor.
To be absolutely certain:
1. Consult a repair manual: A repair manual specific to your 2001 8.1L engine (including the make and model of the vehicle or vessel) will have a detailed diagram showing the location of all sensors. This is the most reliable method.
2. Use online diagrams: Websites like AutoZone, Advance Auto Parts, or others selling auto parts often have online diagrams. Search for your vehicle's details to find the diagram.
Remember to disconnect the negative battery terminal before working around any sensors or electrical components.