1) Check the "manufacturing date".
You can even do this when your xbox is stick packed 'cause there's a small whole at the back of the xbox package where you can read the serial & manufacturing date.
The manufacturing date (MFG. DATE) is in international format : year-month-day
If the manufacturing date is prior August 2002 : you have a v1.0
If the manufacturing date is August 2002 or later : it can be a v1.0, v1.1, v1.2, or v1.3 ... try another test.
2) Check your Xbox bios/kernel version.
Goto the "settings" option in the XBOX menu and select "Sytem Info". You will see lots of legal text.
At the end of the txt you will see something like
K: 1.00.xxxx.01
D: 1.00.xxxx.01
If you don't see this you still have one of the first xbox versions (3944 - USA only) , so it's a v1.0
Check the K number.
If the xxxx = 4034 or 4036 or 4627 you have a v1.0
If the xxxx = 4817 or 4972 you have a v1.1
3) Check if there's a FAN on the GPU
Open your xbox and remove the HDD & DVD-ROM. If there's a fan on the gpu heatsink (next to cpu) you have a v1.0 , if there's no fan you have a v1.1
4) Check if there's a USB pcb
Open your xbox and check the USB chipset. If it's a separate pcb you have a v1.0 , if it's on the xbox motherboard you have a v1.1
5) LPC points
If the LPC points are filled it can be a v1.0 OR v1.1 , if they are empty it's a v1.1
6) Serial Numbers
Check digits 8 and 9 on your serial number.
14, 20, 21, 22 = 1.0
23 = 1.0 or 1.1
24, 25 = 1.1
30 = 1.2
31 = 1.3