Install apt-file, update its database:
sudo apt-get install apt-file
sudo apt-file update
Now you can use apt-file to look at the files in a package, for example
apt-file list openbox
Note that apt-file will list the contents of all packages that match the search pattern, in this case it will show the packages openbox, openbox-dev and openbox-themes.
For more info on apt-file have a look at