(not sure why this is tagged popular now, but I have an answer for those of us who do not have full admin rights. )
See also message 89515 from 2014 in geonet.
The fulfillment ID can only be linked to the ESU no. by calling ESRI support. The FIDs are really long, so I would
take a screenshot of each one. The ESUs should show up in MyEsri if you are authorized to see licensing for your account.
However, if the single-use license systems still have the software installed, it does not take administrative
rights to deauthorize them. Users can do this themselves; or a sysadmin could log in as them and do it. They will lose ArcMap until they get a new concurrent license set up, which again a sysadmin could set up for them pretty quickly if the license server is up and running.
As they are deauthorized, the ESUs/licenses will disappear pretty quickly (an hour?) from MyEsri, so you can infer who they belonged to if you have people e-mail you when they have deauthorized. But if you want to do the deauthorization without disturbing users, I don't know the best way to do this.
ESRI can deauthorize/release licenses for you, but they should only need to do this if someone uninstalled the software or if a system dies before a single use license is deauthorized. So make sure everyone is trained not to do the former; the latter situation is inevitable.
Correction: Technically, software can be uninstalled without losing the ESU, but reinstalling would require finding a good unused ESU. And, when you don't know who has which license, that can be tricky.