'lesspipe <package>.deb' or 'less <package>.deb'
will show file locations for a non installed package.
'dpkg -L <package>' : locations, an installed package.
Or you can look into the file list online :
Example
http://packages.ubuntu.com/lucid/leafpad
.. and find the link 'list of files' :
→ → Last section on the page
http://packages.ubuntu.com/lucid/i386/leafpad/filelist
And finally : You can create a local "all packages file list" :
1) sudo apt-get install apt-file
2) apt-file update ( will download the lists.)
3) apt-file list <package>
... will then display the file list for the package.
Can also do: apt-file search <file> and apt-file search path/to/<file>
...