IIRC the voltage output across a narrow band O2 will look like a sine wave if the system is working correctly, but that means you have to have a system that is toggling between too rich and too lean and that means you have to have good (not clogged) injectors, right? If that's the case putting a VOM across the O2 won't really tell you anything.
One thing I don't like about the 3FE is there is no service port on the fuel rail for a pressure test guage. I have a nifty Kent-Moore injector test set that will drive the injectors one at a time with a calibrated pulse train. You just pressurize the system with Key-On, Engine Off, get a pressure reading, send the pulse train and read pressure again. Repeat for each injector and if there is a difference in pressure readings on any injector you have a clogged one.
IOT test the Toyota injectors I have to pull them out and put them on a Chevy fuel rail. In a Chevy I can test all 8 injectors in place in less than 5 minutes without getting my hands dirty. I don't know why Toyota skimped on a simple schrader valve for a fuel rail service port.