If you want to see what the current Startup Disk is on your macOS installation, you can certainly go to System Preferences > Startup Disk.
But if you want to use the terminal instead of the GUI, this command will return the current Startup Disk:
bless --getBoot
If a Startup Disk is set, you'll see something like this:
/dev/disk0s2
If no Startup Disk is set, you'll see this error message instead:
Can't access "efi-boot-device" NVRAM variable
or this one:
Could not interpret boot device as either network or disk
Can't interpet EFI boot device
And, yes, there's a typo in the error message (as of macOS 10.12.6, anyway). That should say Can't interpret instead of Can't interpet.
Can't interpet EFI boot device