How can I tell what version of SMB is negotiated when I connect to a share from macOS? I have quite a few file servers at work that serve up files over SMB/CIFS. 

How can I tell what version is negotiated when someone says that I need to be sure I'm connecting over SMB version 3 to get best performance?
The smbutil command can dump the attributes for all shares or you can target one particular share and parse out the version string:

smbutil statshares -a


In the case of the listing below:

site-test-mbp01:~ me$ df
Filesystem                                           512-blocks       Used  Available Capacity  iused               ifree %iused  Mounted on
/dev/disk1s1                                          975210568  368841648  602865928    38%  1808606 9223372036852967201    0%   /
devfs                                                       706        706          0   100%     1222                   0  100%   /dev
/dev/disk1s4                                          975210568    2097200  602865928     1%        2 9223372036854775805    0%   /private/var/vm
map -hosts                                                    0          0          0   100%        0                   0  100%   /net
map auto_home                                                 0          0          0   100%        0                   0  100%   /home
/dev/disk3s1                                         7813365344 1387760392 6424657392    18%    28307 9223372036854747500    0%   /Volumes/apfs
com.apple.TimeMachine.2018-03-29-120539@/dev/disk1s1  975210568  367917520  602865928    38%  1809465 9223372036852966342    0%   /Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/site-test-mbp01/2018-03-29-120539/Macintosh HD
com.apple.TimeMachine.2018-03-23-083810@/dev/disk1s1  975210568  348612592  602865928    37%  1755978 9223372036853019829    0%   /Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/site-test-mbp01/2018-03-23-083810/Macintosh HD
//me@file-nasuni/abc                              2147409920     227552 2147182368     1%   113774          1073591184    0%   /Volumes/abc


To target the Nasuni share "abc"

smbutil statshares -m /Volumes/abc/|grep VERSION|awk '{print $2}


The command shows I'm negotiated a SMB_3.02 connection to that storage appliance share.