FYI: for now we only use systemd-boot when booting ZFS as root, other cases may follow though in the future.
There are a few differences which can be told:
* The boot loader looks pretty different, GRUB is well GRUB, by default blue menu and a grub written somewhere. systemd-boot is just very plain black.
* On the booted system you can normally tell by checking if /etc/kernel/pve-efiboot-uuids
exists, if not you're highly probably using GRUB
In general you can check if you booted with UEFI mode (independent of which bootloader) by looking if the path /sys/firmware/efi
is populated, else it's empty or doesn't exists at all.