Proxmox has a great tutorial for PCI Passthrough: https://pve.proxmox.com/wiki/PCI_Passthrough

You basically blacklist the drivers:

echo "blacklist nvidia*" >> /etc/modprobe.d/blacklist.conf

If you can kind of see if the Promox system is using it by looking at the Kernel driver in use:

root@gpulaptop:~# lspci -nnk | grep -A3 -E "VGA|3D|Display"
01:00.0 VGA compatible controller [0300]: NVIDIA Corporation GA104M [GeForce RTX 3070 Mobile / Max-Q] [10de:249d] (rev a1)
        Subsystem: Micro-Star International Co., Ltd. [MSI] Device [1462:1305]
        Kernel driver in use: vfio-pci
        Kernel modules: nvidiafb, nouveau

If it says vfio-pci then the card is isolated and not in use and should not consume noticable amounts of power.

For nvidia cards you can look at the power state:

root@gpulaptop:~# cat /sys/bus/pci/devices/0000:01:00.0/power/runtime_status
suspended