If the directory /sys/firmware/efi/efivars
exists, then your systems booted with UEFI.
If the command zfs get all /
return a list of lines about something like rpool/ROOT/pve-1
, you are running Proxmox from a ZFS file system.
According to the post above, if both are the case, you will have booted with systemd-boot (which has a different looking boot-menu than GRUB).
You can also use cat /proc/cmdline
and see if it matches grep 'CMDLINE_LINUX=' /etc/default/grub
(GRUB) or it matches cat /etc/kernel/cmdline
(systemd-boot).