Where in the file system is my shared folder? (AKA: Where is /Volume, and Why Can't I Open It in Finder?)
The folder you are looking for is under /Volumes in your file system. /Volumes is sometimes called a hidden file in MacOS parlance.

Using Finder with Hidden Files:

There are at least two ways to expose hidden files in Finder:   


Use the Finder menu to navigate to arbitrary folders:  Go, Go to Folder..., (ALTERNATIVELY: shiftcommandG), and then enter /Volumes in the text box. Of course this method requires that you know the name of the file/directory you're looking for, but once that's known this will get you there.
If you're running macos Sierra (or later), you can view all hidden files/folders in Finder like so: shiftcommand. (shift-command-period). Repeating this sequence will hide the hidden folders again. 


Note: To see /Volumes using the 2nd method, you must position Finder at the proper place in the file system; i.e. at the Macintosh HD level (selected from Finder's sidebar if you have it enabled!). 

Once Finder has listed the hidden file/folder of interest, use the menu (File, Get Info) or shortcut (commandI) to yield the info window with lots of handy information. 

At this point, your questions re. Finder and hidden files have hopefully been answered. Now let's move on to the rest of your question re. how to display usage stats from the command line:

Hidden Files from the Command Line:

Open a terminal window (bash is the default macos shell). Just like every (?) other *nix system in the universe, the file system root is simply known as /. So, let's see what's in /: 

$ ls -l /
total 13
drwxrwxr-x+ 80 root  admin  2560 Mar 17 18:42 Applications
drwxr-xr-x+ 64 root  wheel  2048 Feb  4 01:43 Library
drwxr-xr-x   2 root  wheel    64 Feb  4 01:39 Network
drwxr-xr-x@  5 root  wheel   160 Sep 21 05:05 System
drwxr-xr-x   7 root  admin   224 Feb  4 01:39 Users
drwxr-xr-x@  4 root  wheel   128 Mar 17 22:48 Volumes
drwxr-xr-x@ 37 root  wheel  1184 Feb  8 02:57 bin
drwxrwxr-t   2 root  admin    64 Feb  4 01:39 cores
dr-xr-xr-x   3 root  wheel  4512 Mar 17 12:17 dev
lrwxr-xr-x@  1 root  wheel    11 Feb  4 01:37 etc -&gt; private/etc
dr-xr-xr-x   2 root  wheel     1 Mar 17 22:10 home
-rw-r--r--   1 root  wheel   313 Aug 18  2018 installer.failurerequests
dr-xr-xr-x   2 root  wheel     1 Mar 17 22:10 net
drwxr-xr-x   3 root  wheel    96 Sep 26  2016 opt
drwxr-xr-x   6 root  wheel   192 Feb  4 01:39 private
drwxr-xr-x@ 64 root  wheel  2048 Feb  8 02:57 sbin
lrwxr-xr-x@  1 root  wheel    11 Feb  4 01:37 tmp -&gt; private/tmp
drwxr-xr-x@ 10 root  wheel   320 Feb  4 01:44 usr
lrwxr-xr-x@  1 root  wheel    11 Feb  4 01:38 var -&gt; private/var
$


Listed here are all of your files and folders, including links, and the hidden files, including /Volumes. If this makes you wonder about the conventional wisdom (or current practice) re. "ease of use" for Graphical User Interfaces (GUI), then you are on the right track :) But I digress... 

Now, take a look to verify that your mounted shares are listed under /Volume. In your case, assuming your raspberrypi share is the only one mounted, you'll see something similar to this: 

$ ls -l /Volumes
total 32
lrwxr-xr-x  1 root    wheel      1 Mar 17 12:17 Macintosh HD -&gt; /
d--x--x--x+ 2 macusr  wheel     64 Mar 17 12:18 passport2tb
drwx------  1 macusr  staff  16384 Mar  4 23:56 passport2tb-1
$ 


The first line is a link showing the mapping from the Macintosh HD to the file system root /. The second line will show your mounted network drive, but note the permissions - you won't be able to use this one. It's apparently an artifact. I don't know why it's there other than to set the executable permission bit for the folder; perhaps someone else can offer an explanation? At any rate, while we needn't be overly concerned about why it's there, automating this task will require that we identify the correct share name. 

Automating the process via bash script:

The correct share name can be found as follows:

$ ls -l /Volumes | grep drwx | head -1 | awk '{print $9}'
drwx------  1 macusr  staff  16384 Mar  4 23:56 passport2tb-1 
passport2tb-1
$


To recap: grep will filter the output of ls -l to get the actual share (instead of the artifact), head -1 will filter duplicates (which may show up as passport2tb-2, etc) and awk '{print $9}' will get the 9th field in the string which is the share name. If you have other shares mounted in /Volumes, simply add another grep stage to get the one(s) you're interested in.

Knowing the location of your mounted share will allow you to successfully execute the du command to display the usage statistics of any folder in the mounted share; for example: 

$ du -sh /Volumes/passport2tb-1/data_lib
 75G    /Volumes/passport2tb-1/data_lib 
$


In this case, the folder data_lib folder contains 75GB of data... and this may take a few minutes to tally, even on a fast network. 

Some command line gymnastics that might be useful: 


If you don't want/need the path information in du, use awk to strip it: 


$ du -sh /Volumes/passport2tb-1/data_lib | awk '{print $1}'
75G
$



And if you don't want the "G": 


$ du -sh /Volumes/passport2tb-1/data_lib | awk '{print $1}' | cut -d "G" -f 1 
75
$


To wrap this up in a script: 

#!/bin/bash
SHRNM=$(ls -l /Volumes | grep drwx | head -1 | awk '{print $9}')
SIZE=$(du -sh /Volumes/$SHRNM/data_lib | awk '{print $1}' | cut -d "G" -f 1) 
echo $SIZE


Finally: Save it, name it, make it executable, and add it to your crontab to be executed periodically. 

This answer has gotten long-winded, so I'll leave this as an exercise for the reader.