@TimV-0 is it a 5" integrated PanelDue, or a non-integrated one? If it is non-integrated, what PCB version is it?
If it's an integrated one then you can check whether the backlight inverter is working by measuring the voltage across the capacitor labelled C3 near the top right hand corner (looking at the back) of the PCB. It should be about 17V. Note, it can be difficult to get a good connection to SMD components when using a multimeter, because of oxidation on the surface of the lead-free solder.