In fairly low ambient light conditions. Have someone do the glow plug sequence. If enough glow plug current is flowing the squiggly wires between the plugs will visually appear to be very hot. If they do it indicates the glow plugs are probably working well enough.

After this if the wires indeed past the test. You could either spray a little wd40 down the throat of the air filter with the cover off after a glow cycle. Or loosen one of the nuts on an injector to see if fuel is present when cranking.