In order to validate that all NBS servers preboot environment are in it, would be very useful to have a report similar to the "Preboot configuration status" page that is available under "Settings > Deployment > Preboot configuration", selecting a preboot configuration and click on Status (due to privacy note that the server names have been removed).

Deployment Solution 7.5 and above

You can create a report based on the following SQL query:

SELECT _ResourceGuid _ItemGuid, _EventTime EventTime, TSName ServerName, ConfigName, OEMExtension, PrebootType, Architecture, status, ActionType
FROM dbo.Evt_PXE_Image_Status s
WHERE _eventTime IN (SELECT MAX(_eventTime) FROM dbo.Evt_PXE_Image_Status GROUP BY TSGuid,ConfigName )
ORDER BY _eventTime desc

When running you will get something simliar to this (with the server names on your enviornmnet):