If you have the single diode set-up such as the '06 then the diode is used to isolate the clutch interlock switch so pulling the clutch in doesn't turn on the neutral light and you can use this function to help isolate your problem.
If the diode is reversed or open circuit or the neutral switch bad then you won't be able to start the engine with the bike in neutral without pulling in the clutch.
If you can start the engine with the bike in neutral then the diode is either normal or short circuit but more importantly, the neutral switch is operational and the neutral light itself or the wiring associated with it is bad.
If you can't start the bike in neutral then take note of the position of the diode (it has a symbol similar to this -|< ) and swap it 180 degrees. Turn the ignition ON and pull the clutch in. If the neutral light itself is OK this action should turn it on. If you are really having a bad day then having two things faulty (ie switch and diode) will really cloud the issue. If you have any suspicion about the diode then for testing the light simply substitute a fuse. This will allow you to test the bulb with the clutch pulled in. This will not harm anything but don't leave it this way because the clutch switch turning the neutral light on will be a nuisance.
The other really unlikely (but possible) scenario is that you have both the neutral switch and the bulb in the instruments bad.
However, before you get too worried the simple checks above should let you better isolate the issue without any measuring equipment.
