Neiter of the XT's have fuel gauges (2005-2007 models) not sure about the 2008 though.
They just have a little orange light next to the speedo that blinks whenever the tank gets to 2 litres, or if you ride above 60mph for more than 30 seconds, haha. I found the best way to tell how much fuel you have left is to reset your milometer when you fill up and count the miles before you run out.