Hi poison2015, are you asking how to find out which physical GPU is GPU0 in your rig?
Stop the Miner from the web GUI (Miner Actions, Stop Miner)
Start a SSH shell to your rig and login as user, type gpu-fans-find [GPU number]
Example: gpu-fans-find 4
(This will stop all fans on all GPUs and only spin-up the fans on GPU4 for about 10 seconds)
Probably quickest and easiest to just reboot your rig when you have finished labelling your physical GPUs.