I have just installed a new expander board.
 How do I check to find out if it has already been reworked?      

Rev. levels can be found using the prtfru command.  Below are 2 examples that show the
difference as components are revised.

Expander board fresh that has not been modified.
$ prtfru "/frutree/chassis/CP/ex16?Label=ex16/EXB"
SEGMENT: FD
   /InstallationR (1 iterations)
   /InstallationR[0]
   /InstallationR[0]/UNIX_Timestamp32: Sun Nov 18 14:25:43 MST 2001
   /InstallationR[0]/Fru_Path: /chassis/cp/ex16
   /InstallationR[0]/Parent_Part_Number: 5014936
   /InstallationR[0]/Parent_Serial_Number: 396049
*==>   /InstallationR[0]/Parent_Dash_Level: 10  
   /InstallationR[0]/System_Id: A84A40
   /InstallationR[0]/System_Tz: 420
   /InstallationR[0]/Geo_North: 0
   /InstallationR[0]/Geo_East: 0
   /InstallationR[0]/Geo_Alt: 0
   /InstallationR[0]/Geo_Location: TZ=US/Mountain
   /Soft_ErrorsR (0 iterations)
SEGMENT: PE
........deleted for readability.

Expander board fresh that has been modified.
$ prtfru "/frutree/chassis/CP/ex1?Label=ex1/EXB"  
/frutree/chassis/CP/ex1?Label=ex1/EXB:
SEGMENT: FD
   /InstallationR (2 iterations)
   /InstallationR[0]
   /InstallationR[0]/UNIX_Timestamp32: Tue Nov  6 16:53:52 MST 2001
   /InstallationR[0]/Fru_Path: /chassis/cp/ex3
   /InstallationR[0]/Parent_Part_Number: 5014936
   /InstallationR[0]/Parent_Serial_Number: 429697
   /InstallationR[0]/Parent_Dash_Level: 11
   /InstallationR[0]/System_Id: A809FA
   /InstallationR[0]/System_Tz: 480
   /InstallationR[0]/Geo_North: 0
   /InstallationR[0]/Geo_East: 0
   /InstallationR[0]/Geo_Alt: 0
   /InstallationR[0]/Geo_Location: TZ=US/Pacific
   /InstallationR[1]
   /InstallationR[1]/UNIX_Timestamp32: Tue Feb 12 11:37:48 MST 2002
   /InstallationR[1]/Fru_Path: /chassis/cp/ex1
   /InstallationR[1]/Parent_Part_Number: 5014936
   /InstallationR[1]/Parent_Serial_Number: 396049
*==>   /InstallationR[1]/Parent_Dash_Level: 10
   /InstallationR[1]/System_Id: A84A40
   /InstallationR[1]/System_Tz: 420
   /InstallationR[1]/Geo_North: 0
   /InstallationR[1]/Geo_East: 0
   /InstallationR[1]/Geo_Alt: 0
   /InstallationR[1]/Geo_Location: TZ=US/Mountain
   /Soft_ErrorsR (0 iterations)
   /ECO_CurrentR
   /ECO_CurrentR/UNIX_Timestamp32: Tue Nov  6 17:49:54 MST 2001
   /ECO_CurrentR/Firmware_Revision: 
   /ECO_CurrentR/Hardware_Revision: 51
**==>   /ECO_CurrentR/HW_Dash_Level: 16
SEGMENT: PE
........deleted for readability.

*==> This is the revision of the board when it left manufacturing

**==> This is the current board level if it has been reworked.