Can you tell if updates came from OS X caching server?
The caching server will print to log when a client requests an update. This log is located at:

/Library/Server/Caching/Logs/Debug.log


The following is the anatomy of a request which was served by the caching server's cache:


Client requests update package

Got request for host = http://swcdn.apple.com/{{REQUEST}}

Find cached package in server data

Initializing asset handler for http://swcdn.apple.com/{{REQUEST}} (path = /Library/Server/Caching/Data/{{UDID}})

Check/load asset in cache

Cached asset length = {{LENGTH}} MD5={{CHECKSUM}} last modified {{DATE/TIME}}
Extents loaded from disk: {{DATA}}
Data already cached for asset http://swcdn.apple.com/{{REQUEST}}, issuing If-Modified-Since request
Info loaded: file length = {{DATA}}, reader = {{DATA}}

Serve asset to client from cache

{{BYTES}} bytes served, {{SERVED_BYTES}} from cache, 0 downloaded


This 0 downloaded is what shows the update was entirely served by the caching server.
Finish

Outgoing connection finished