Nowadays most of the applications for Mac OS X come in the .app format and it's easy to see the package contents before using them (just right click and select Show Package Contents). Most of these even come from the App Store, so you don't have to worry that they will install something you don't want on your shiny Mac.
That said, there comes a time when an Application simply can't be contained only in .app as it needs to install extra services. These are the ones that needs to start with your Mac or to run in the background in order for the app to function properly.
An example for these is the Google Voice and Video installer (that is needed for Hangouts to work) or the amazing TeamViewer screen sharing program that allows untended access to your computers.
An example for these is the Google Voice and Video installer (that is needed for Hangouts to work) or the amazing TeamViewer screen sharing program that allows untended access to your computers.
Both of these come in a .pkg file format and don't have the Show Package Contents option available.
Here is how to see what these package applications will install before installing them:
1. Run the .pkg package installation file as usually by double clicking on in
2. Now when the installer shows, don't press continue
3. Instead go to File and choose Show Files
This will show you all the files the package is going to install and also the location that it will install them in. It's a good measure to check what launch services every package installs before hand, especially in the LaunchDaemons and LaunchAgents folder, because they will start up always with your Mac. This is also useful if you would like later to remove any of them manually, since now you know where to find them.
Thanks goes to the users across the Internet forums like alexbates who contributed their ideas and experiences to solve this kind of issues.