Using Spotlight index on a file share on Server.app
It looks like mdutil can see that server search is enabled:
$ sudo mdutil -s /Volumes/YourNetworkDrive
  /Volumes/YourNetworkDrive:
           Server search enabled.

Also the mdutil manpage has this information for the -p option:
-p  Spotlight caches indexes of some network devices locally. 
This option requests that a local caches be flushed to the
appropriate network device.

However, for me this does not seem to work, and it doesn't show remote search results anywhere when using SMB network mounts (This is in macOS Sierra).
There is one officially supported way to query remote Spotlight indexes, by using the Apple Remote Desktop application. Apple Remote Desktop is not free, but can be purchased through the Mac App Store ($79.99).
However, this a separate app and it does not integrate the remote index with local Spotlight searches. It works well though.
From the features page:

Remote Spotlight search
Leveraging the revolutionary Spotlight technology in OS X, the Remote
Spotlight capability in Apple Remote Desktop can perform
lightning-fast searches on remote client systems. Summary results for
each client are updated instantly as results are returned. View
details on results, or refine searches even further using additional
qualifiers. Results can be viewed on remote client systems, copied
back to your administrator system, or deleted.

A screenshot from the product page: