How can I view the signer of a bundle?
If you have the optional command line tools that XCode will install for you, codesign is the tool of choice.

Using the Terminal app:

$ codesign --display -vv /Applications/GarageBand.app/
Executable=/Applications/GarageBand.app/Contents/MacOS/GarageBand
Identifier=com.apple.garageband
Format=bundle with Mach-O thin (i386)
CodeDirectory v=20100 size=24489 flags=0x200(kill) hashes=1218+3 location=embedded
Signature size=4169
Authority=Apple Mac OS Application Signing
Authority=Apple Worldwide Developer Relations Certification Authority
Authority=Apple Root CA
Info.plist entries=27
Sealed Resources rules=14 files=2021
Internal requirements count=2 size=500


And a non-Apple signed app..

$ codesign --display -vv /Applications/iTerm.app/
[snip]
Authority=Developer ID Application: GEORGE NACHMAN