How to tell if if a GPU is dead???

Last night I had 4 Sapphire Nitro+ RX480s running with a modded bios. The rig was stable for over 24 hours. When I woke up this morning one of the cards was not working. I verified it is not an issue with the riser or motherboard. The motherboard will not recognize the GPU. The sapphire light still comes one when the machine is powered but thats about the extent of life I can see. The card does not smell burnt or anything.

Any other ideas on what I can do?

Thanks.