To find a checksum hash file or SHA-1 digest code for an OS X upgrade or downloaded .dmg file:

1 – Download the .dmg

2 – Launch Terminal and at the command prompt type open sha1 and the path to the dmg or just drag and drop the dmg into the Terminal after the command

 openssl sha1 [path-to-file]

For example to find OSX 10.8.2 upgrade checksum SHA1 digest