Xbox v1.0-v1.5 have MFG Date (manufacturer date) up to may 2004. Consoles with a later MFG Date are 1.6 version or higher. To be more certain you can check your video chip brand. Open your console and check your board, the video chip is located right behind the RGB/AV cable input. If you have Xcalibur written on it you have a version 1.6!
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If you bought your Xbox console right before they stop selling it, you can be sure you have the 1.6 version unless the store you bought it from keep old stock from the beginning of 2004 =)
You can also find out which version you have by checking the serial nr on your xbox console, examples :
Serial XXXXXXX 30205 = v1.2
Serial XXXXXXX 31205 = v1.3
