Below you will find an easy to follow guide to help you find your console version. This is a cut down version of an in-depth guide that can be found at xbox scene. All credit goes to the original authors......
As of 14/09/03 there are 5 main versions of Xbox.
IMPORTANT: These tests are just made by examining our xbox consoles. There's NO guarantee these test are 100% correct. Also keep in mind a new version might have been released without us knowing it yet.
1/ Check the "manufacturing date".
You can even do this when your xbox is stick packed 'cause there's a small hole at the back of the xbox package where you can read the serial number.
Serial Number v1.0 : XXXXXXX 20205 (20, 21 = v1.0)
Serial Number v1.1 : XXXXXXX 24205 (24, 25 = v1.1)
Serial Number v1.2 : XXXXXXX 30205 (30 = v1.2) Serial Number v1.3 : XXXXXXX 31205 (31 = v1.3 )
Serial Number v1.4 : XXXXXXX 33205 (33 = v1.4 ) Date29/07/03
2/ 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 heat sink (next to cpu) you have a v1.0
If there's no fan you have a newer version.
3/ Power connector The power connector has a single range of connectors on v1.0 and v1.1,while the v1.2 has a two.