How to test the oxygen sensors with a multimeter?

you can test if they are going from 0 - 5v from the ecm

This is where LPG systems pick up the signal from

Nice but my LPG system picks O2 sensors signal directly at one of them.

i've just installed an OMVL dream multi point system, looked through rave's wiring diagrams and found that all injectors, oxygen sensors and RPM input can all be taken from the ecm which made the wiring a doddle

IIRC, you need a oscillator scope or whatever it's called (I haven't used one since I switched from EE to business...12 years ago), because they act really fast and you need to be able to measure it quickly, and in a graph-type form.