Which pulley is the idler pulley?

There should be a sticker under the hood somewhere with a picture of the Auto serpentine belt (V-belt) install !
The idler pulley should be in this picture,it keeps tension on your serpentine belt.

There isn't a diagram of the pulleys anywhere. Does anybody know which one the idler pulley is? Also how difficult is it to change?

What year and which engine does you LS have?

-Rod

I see from one of your other posts that it's a 2000 V8. Below is the diagram from the 2002 Factory Service Manual which should be the same as your 2000. You'll notice there are two idler pulleys.
http://i538.photobucket.com/albums/ff342/raschmidt/Automotive/V8Beltrouting.jpg

-Rod