What official government document should a pilot check to determine what magnetic north a VOR was set to when it was commissioned?
The chart supplement does list the magnetic variation of the VORs. Looking under the "radio aids to navigation" section for a nearby airport, it's listed at the end of the line, right after the elevation.


  BATTLE GROUND (H) VORTACW 116.6 BTG Chan 113 N45º44.87´ W122º35.49´
  160º 9.6 NM to fld. 253/21E.


Another source is the radio fix and holding data records from the FAA. Among other information, you can find the magnetic variation in the remarks section, along with the year it was set.


  REMARKS:
  ...
  BTG MAG VAR 1975/E21




It's not official, but the easiest place I've found to get this is to search for the navaid on AirNav.