Information about which beacon(s) an agent has communicated with is not gathered or stored centrally. Agents dynamically prioritize and choose which beacon to communicate with each time a communication is required, and don't have a concept of a "fixed" beacon.
As you've found, info on what beacon is used for any particular download or upload communication can be found in agent logs. I guess if you were really keen and the situation warranted it you could also look at IIS logs on the beacon to see what IP addresses HTTP requests are coming in from, and somehow make use of that information. You would also see hostnames of the computers with the agent installed appearing in these logs in HTTP GET requests for .npl (policy) files and HTTP PUT requests for .ndi (inventory) files.
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