Question about the Canon EOS Digital Rebel XT?

Does this camera show the picture on the screen before you take it or is it only for use through the viewfinder? How can I tell by looking at the product specs?

With most dSLR cameras, including the XT, the screen is only for operating the menus and for looking at pictures that you've already taken.
The exceptions to this rule are a couple of cruddy Olympus cameras and a professional level Canon.

yes that is called a digital camera... digital cameras have a screen

If you have ever looked through the view finder on a SLR it is the same thing you will see through the view finder on a DSLR. They are really much a like in design. It is assumed that photographers know that if they are going to spend over ($1000 in most cases) for a camera and one lens. The clue in the specs is "DSLR".

The mirror has to come up, the shutter has to open and then the sensor (or film) is exposed. With the shutter closed and the mirror back in its resting place, you can press the preview button and see what the sensor saved to your memory card on the LCD screen

before and after you take the picture

Short answer no.

The mirror is blocking the sensor which "live preview" on the LCD takes place when light is on the sensor. You can only use the viewfinder for the Rebel XT to compose a shot.