On Debian-based systems, can one view the man page of packages that aren't yet installed? I want to check what they are capable first, before installing them.
You can use debman to view man pages before installing a package. See: http://www.debian-administration.org/articles/454.
Usually the man pages get installed as part of the package.
Perhaps the best idea is starting your search at the web page of your distribution and looking for specific documenation there. I currently use Ubuntu and a quick search yielded the Ubuntu Manpage Repository. I am sure similar pages exist for all the major distributions.
However, anytime I've quickly looked for a man page, I've just googled '
man page some_command' and found several copies of the Unix/Linux man pages on line. I do this frequently when posting here or on SO and want to provide a reference for some command I'm mentioning in a post.
UNIX ON-LINE Man Pages is a repository of man pages.
And here is another index of On-line UNIX manual pages.
sudo apt-get install manpages-dev
will install the man pages for system and library calls.
Update: As @jasonwryan points out in a helpful comment, the GNU Manuals are available online too.
Update 2: Another useful comment by @user606723 reminds us that there can be different versions of commands/man pages, so while looking at the documentation it would be prudent to note specifics.
A debian developer provides a service named manpages.debian.net where you can access shipped manpages online and can also get a release specific man page.
There is also manpages.ubuntu.com which does the same for Ubuntu.
As this method allows you to access release specific man pages you'll probably get the correct versions for your Distribution.
To the more command-line inclined, there is the option of using a combination of
Add the following to
SURFRAW_graphical="no" SURFRAW_text_browser="elinks -dump"
And you're good to go, e.g.
sr google -l man ls | less
NOTE: The comment by user606723 still holds.