How to identify boot in Mac?

I have a 2010 MacPro Intel running only Ubuntu Studio 16.04. I installed from a USB stick. When I looked on the drive where the OS is installed, under a partition labeled "unknown" was listed boot/grub (or grub/boot, I can't remember the order. I'm not on the machine now.) Does this mean that the MacPro boots using grub and not efi or uefi? Also, when I entered 'grub' on the terminal, it returned a message stating the grub was not installed with how to install it. Is this machine booting using grub, or is there some way to tell what the boot manager is? Would reFind help anything? I'm planning to set up a dual boot system, and want to know what I may have to change.

Thanks. :confused: