determining root mounting point from GRUB command line prior to boot

[Blue Upsilon]

Hi there,

I use Ubuntu 18.04.
When I boot from GRUB command line, is there a way to determine what the root 
mounting point will be, before booting?

i.e., say I know that my partition that has Ubuntu is  (hd0,gpt4).
so then I do:     set root=(hd0,gpt4)
and then I happen to know that the correct root mounting point is   
/dev/nvme0n1p4
so then I know to do:    linux /vmlinuz root=/dev/nvme0n1p4
and then i do     initrd /initrd.img     and  boot

but like what if i don't know ahead of time that    /dev/nvme0n1p4   is what I 
need?
(this problem comes up when:   say i'm booting from a backup external disk, and 
i don't know whether it will be /dev/sdb2  or whatever.... etc..)

Thanks,
Blue Upsilon