There is a code on the top of the heads right below the injectors.
USDM 2.0 WRX's are LHS20 and RHS20. The jdm verison 6 heads I have are stamped with the same numbers.
I recently did a version 8 swap for someone and the casting was different but I can't remember the actual numbers.
I found this thread when I type in LHS20 http://forums.nasioc.com/forums/show...ighlight=LHS20