Verify / Check validity of Mac OS X Applications and Packages (.pkj) Digital Signatures

How to verify the authenticity of manually downloaded Apple software updates
( https://web.archive.org/web/*/https://support.apple.com/en-us/HT202369 )

Verifying that a Package is Signed
( https://web.archive.org/web/20150719094756/https://jamfnation.jamfsoftware.com/article.html?id=299 )

Code Signing Tasks
( https://web.archive.org/web/*/https://developer.apple.com/library/mac/documentation/Security/Conceptual/CodeSigningGuide/Procedures/Procedures.html )

OS X Code Signing In Depth
( https://web.archive.org/web/*/https://developer.apple.com/library/mac/technotes/tn2206/_index.html#//apple_ref/doc/uid/DTS40007919-CH1-TNTAG211 )