In my case I want to do
aptitude install wireless-tools but want to know which files will be installed and where in the filesystem before I install the package.
You can go to packages.debian.org and search for the package you're interested in. Towards the bottom of each package's page, there's a link labeled
[list of files] (one for each architecture) that will take you to the list of files to be installed. For example, here's the list of files for Python.