I am not too sure on how you can tell the difference in the US, but over here in the UK there are codes on the back or side of the game:

GTA: LCS (Pre 2.6 version):
ULES-00151#

GTA: LCS (Patched 2.6 Version):
ULES-00151#2

It is the first code that you need if you want homebrew on 2.6. So, i don't know, but maybe you can purchase a copy of GTA on an English Website

EDIT: not entirely sure if it has already been mentioned, but you can only downgrade if you do not have a TA-082(think its that) motherboard. To check if you have 1. Open up the UMD cover case, look at the top right of the opened up space, if there is a code, there is a chance that you have a TA-082 board. If there is nothing there, it should be safe to downgrade, but it still doesn't come without risks!!