GPU rendering is already in use with the parameters you mentioned, I discovered it when trying to figure out why 4k outputs were rendering as 2k images.
I'm not asking how to enable GPU rendering, I'm asking how to know which GPU is selected for rendering, or in the case of multiple GPUs priority/distribution of render tasks to GPUs. Eg, if I want to render 10,000 times, is each sbsrender call going to task the iGPU(which is dx10 capable) each time and thus render slower than a more powerful dGPU like an Nvidia 1080?
Can I alternatively blacklist the iGPU?