from http://msdn.microsoft.com/en-us/library/bb820957(office.12).aspx you will find that outlook save the last update time of an OAB in the Windows registry: HKCU\Software\Microsoft\Exchange\Exchange Provider\OAB Last Modified Time. but The type of this registry entry is REG_BINARY. The data is 8 bytes in size. You can convert the data to a 64-bit FILETIME structure specifying a Universal Coordinated Time (UTC) value that Outlook last downloaded the OAB files from the Exchange server to the client computer.
here is an ps script:
jeff provided a VBS script to help you change the value as an easy read type at http://www.expta.com/2010/06/vbscript-to-tell-last-time-oab-was.html.