Most image viewers and editors will allow you to view the EXIF info included in the photos. Some do include more and others include less of the information contained in the EXIF data. Most Adobe products tend to strip the "maker notes" section of the EXIF when exporting the image to another format (e.g. .dng, .jpeg, etc.). They may ignore the serial number when displaying EXIF info even when the data is still there in an imported image.

If you can't get the serial number to display using the "Camera Data" in Photoshop Elements 10, you have a few other options.

Digital Photo Professional is included on the disc of applications shipped with every Canon DSLR. It is a fairly straightforward process to view the serial number of an EOS camera from the EXIF of an image made with that camera using DPP.

Select an image, click the Info button and a new window will open displaying the EXIF info. Scroll all the way down to the bottom and the Camera Body Number will be displayed as the next to last item.

A free image viewer that displays the serial number in the EXIF info is Irfanview. (To open and display raw .cr2 files you will need to install the main program and the plugin. Click on the "plugin" link on the main page and follow the instructions for CRW under the *Plugins updated after the version 4.0.)

With Irfan view, just open an image, click the info button (the blue circle with a script "i" in the middle), then click on the EXIF info button in the lower left corner of the information window.

Scroll about halfway down and you will see the camera's serial number.

Note: Some EXIF viewers may display some Canon Camera Body Number/Serial Number values as a hybrid hex/decimal notation. The actual value recorded in the EXIF "maker notes" section for at least some Canon cameras seems to be encoded in such a hybrid format. When I wrote this answer in early 2014 Jeffrey's EXIF viewer displayed the undecoded hybrid number for an EOS 50D, a 7D, and a 5D Mark II. By late 2015 when this answer was written, Irfanview displays the same number stamped on the body for a 7D Mark II and 5D Mark III. For the same 50D, Irfanview still displays the "Serial Number" as "1520708485 (5AA411141)". The number stamped on the bottom of the body is the first number, while the number in parenthesis is the encoded hybrid value that actually appears in the EXIF Info. Images from the older 7D and 5D Mark II also displays both values, while images from the newer 5D Mark III and 7D Mark II only display the stamped serial number with no "encoded" hybrid hex/decimal value in parentheses.