**Update** I have since found a better way to do this as this method actually doesn't work during imaging due to the limited WMI options in WinPE. Please check out my updated post: http://nuttyrat.blogspot.ca/2013/07/how-to-configure-timezones-dynamically.html
I've been trying to find this for an extremely long time!!!
Just a simple WMI condition
Select * from Win32_NTDomain WHERE ClientSiteName LIKE "%ADSiteName%"
HUGE thanks to this site: