Test GPU without frying motherboard

[Aeder]

I recently upgraded my build to an ITX case. I got myself a new motherboard and although my GPU was hanging pretty close to a wall/separation thing, it ran at normal temperatures, even in games. 

Yesterday, while I was tinkering in blender (nothing huge), my screen suddenly turned black and quite a lot of smoke started coming out of my PC. I immediately turned everything off, but damage was already done and it wouldn't turn on again (nothing happened). 

Today I took everything apart and looked for anything burned, to no avail (even under my GPUs backplate). All components smell of burnt plastic, but I think the GPU does the most. So I took out the GPU and disconnected all unnecessary components, but yet again, nothing turned on. Thinking the PSU might be the culprit, I tested it with a paperclip and it worked (or at least the fan did). Finally, I replaced the motherboard (using my old one) and everything worked fine. So I'm guessing my ITX motherboard fried somehow. That leaves the GPU. Considering it smells the most and the new motherboard broke, is it safe to test my GPU with the old motherboard, or will it fry it as well?

Specs:
PSU: Seasonic G-550
New motherboard: ASRock H110M-ITX
Old motherboard: ASRock H170A-X1/3.1
CPU: Intel core i5 6600 (w/ stock cooler)
GPU: 8 GB Sapphire Radeon R9 390X Nitro
RAM : 2x 16GB G.Skill Ripjaws F4-2400C15D-16GVR
Storage: 1 Tb Toshiba HDD and 512 GB SanDisk SSD

TL;DR: new motherboard broke. I want to test my  GPU with another board but don't want to break it as well, in case the GPU is the reason my new board fried.
 

[Jurrunio]

I wont risk it. I mean, technically  you can take off the card's cooler and observe burnt marks (considering it poured smoke, something must have burnt), but just because you dont see the culprit doesnt mean it's fine.

[AskTJ]

Have you ruled out the power supply?