How to Deauthorize single use license, don't know ESU numbers

39230 - Deauthorize one or more of multiple concurrent or single-use licenses has a nice clear recipe for removing licenses from an end users machines. Excerpt:

Single-use licenses

1. To deauthorize a single-use license, verify that SoftwareAuthorization.exe exists in "C:\Program Files\Common Files\ArcGIS\bin".
2. Open a command prompt window, and change the path to C:\Program Files\Common Files\ArcGIS\bin
3. Run the following command to deauthorize the software silently:
   
   Code:

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   SoftwareAuthorization.exe -return ESU123456789,ESU987654321 /s

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It's missing one critical piece though: how does one get the list of ESU numbers authorized on that machine?