This page goes hand-in-hand with the "Shoot JPEG or TIFF?" article.
The photo below identifies some of the JPEG artifacts. JPEG artifacts are littered throughout the photograph. You can download the photo, load it up in your photo editing software, and looking for them under a magnifying glass (zoom).
In the photo above, you'd have to get really close to the photograph to see the artifacts. Even then, you'd still have to be slightly experienced with what a JPEG artifact looks like.
You can easily see the artifacts when we zoom in to the photograph by 2-to-1, as shown in the photo below. See all the artifacts?