how do I know what files are installed by an installer program? How can I find out what files are installed by an installer program?  Is there a program that can trace that during an installation of a new app?
You can press command-I in an installer window to see a list of files that would be copied by the installer:



You can also use lsbom:

lsbom bundle.pkg/Contents/Archive.bom
lsbom $(pkgutil --bom nonbundle.pkg)


If you have already installed a package, you can use pkgutil --files or lsbom:

pkgutil --files com.tapbots.TweetbotMac
lsbom /var/db/receipts/com.tapbots.TweetbotMac.bom