How to determine which GPU is assigned to dom0
Run the command in Dom0:
[root@monet ~]# head -n 1 /dev/vga_arbiter
take the text highlighted in red and use it in the below command
[root@monet ~]# lspci -s 0000:01:03.0
01:03.0 VGA compatible controller: Advanced Micro Devices [AMD] nee ATI ES1000 (rev 02)
The result you get should tell you which card is allocated to dom0, above it was an AMD GPU.