OS X doesn't store your password, so you cannot read it from some file or database.
What you can do is perform a log-in of some kind, e.g. using
su myusername. If you don't want to open a shell, you can always close the opened shell immediately by
su myusername -c exit.
Alternatively, if you're a sudoer, use
sudo -v to just update your timestamp. It will ask for your password, but not run anything else. This will not work if you recently entered your password for
sudo though. You can invalidate your sudo timestamp using
sudo -k though, so just run that before
sudo -v (and maybe just afterwards, to be safe).