DRM (Direct Rendering Manager) is a subsystem of the Linux kernel responsible for interfacing with GPUs of modern video cards.
ls /sys/class/drm/ -l
returns:
lrwxrwxrwx 1 root root 0 Jun 7 23:08 card0 -> ../../devices/pci0000:00/0000:00:03.1/0000:0a:00.0/0000:0b:00.0/0000:0c:00.0/drm/card0 lrwxrwxrwx 1 root root 0 Jun 7 23:08 card0-DP-1 -> ../../devices/pci0000:00/0000:00:03.1/0000:0a:00.0/0000:0b:00.0/0000:0c:00.0/drm/card0/card0-DP-1 lrwxrwxrwx 1 root root 0 Jun 7 23:08 card0-DP-2 -> ../../devices/pci0000:00/0000:00:03.1/0000:0a:00.0/0000:0b:00.0/0000:0c:00.0/drm/card0/card0-DP-2 lrwxrwxrwx 1 root root 0 Jun 7 23:08 card0-HDMI-A-1 -> ../../devices/pci0000:00/0000:00:03.1/0000:0a:00.0/0000:0b:00.0/0000:0c:00.0/drm/card0/card0-HDMI-A-1 lrwxrwxrwx 1 root root 0 Jun 7 23:08 card0-HDMI-A-2 -> ../../devices/pci0000:00/0000:00:03.1/0000:0a:00.0/0000:0b:00.0/0000:0c:00.0/drm/card0/card0-HDMI-A-2 lrwxrwxrwx 1 root root 0 Jun 7 23:08 renderD128 -> ../../devices/pci0000:00/0000:00:03.1/0000:0a:00.0/0000:0b:00.0/0000:0c:00.0/drm/renderD128 -r--r--r-- 1 root root 4.0K Jun 7 23:08 version
NOTE: This shows that there is only a single card, i.e. card0.
NOTE: Paths like /sys/class/drm/… are just symlinks and may change between reboots.
Adjust the commands accordingly for a reliable result.