Test light or multimeter won't pick up injectors, your looking at a 2-3ms pulse time. Need an oscilloscope to test them correctly.
You may be able to hold a screwdriver to them and crank it and hear them opening and closing. as above, they could be well and truely blocked if they have been sitting for ages.
However, are you sure all the wiring is correct? because of duel fuel the injectors have to be shut off on gas, not familiar with all the LPG systems, but I believe most have a module to trick the
ECU into thinking the injectors are firing, when running on gas, sounds like there might be an issue there?