PC UV fluro lights not working. How to test the inverter of CCFL lights

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I have 2 CC-FL UV tubes powered by one double output inverter lighting up the insides of my PC. 12v in and x? out. Worked fine until I did an upgrade, they worked for a short time but now nothing. I have power going to the inverter but wish to know the procedure to test out put. knowing this is a high voltage out , is this too tricky for a cheap PC add on and just biff, or is there a quick check to see if its the inverter or tubes?
update: believe the output is 680 AC v ± 10%