Disk Utility is Apple's tool for managing disks and disk images in OS X. One component of this program's image handling features is the ability to compute a checksum to verify the image's integrity. Sometimes when you download a disk image from a Web site, the site will mention a checksum for the image that you can compare to a locally computed checksum. If these do not match, then chances are the file was altered from the intended version supplied on the site.
There are a number of checksumming routines commonly used for providing integrity checks for disk images and other files distributed on the Internet, but Disk Utility by default only offers CRC-32 checksums as its sole option, which can be accessed in the Images > Checksum menu. When selected, you can then locate and choose your desired disk image, and have Disk Utility compute and present the checksum result for you.
Having only one option may be a bit limiting, especially since other checksums like MD5 and SHA are commonly used. However, even though Disk Utility only shows CRC-32, it does have hidden support for other checksum routines. To enable this, simply open the OS X Terminal and run the following command:
defaults write com.apple.DiskUtility advanced-image-options 1
After running this command, instead of only seeing "CRC-32" as the checksumming routine, you will have options for MD5, various SHA algorithms, and iPod and Disk Copy algorithms.
As with the CRC-32 checksum, you simply need to select any one of these, and then choose the desired disk image in the Open window, after which the checksum will be computed and presented to you.
To undo this option simply rerun the Terminal command and change the "1" to a "0," and then quit and relaunch Disk Utility.