You look at the post-installation script, which is actually run by dpkg. You can find these in /var/lib/dpkg/info. Such scripts contain the name of the binary package in question, and have the suffix .postinst.
Note that there are also preinstallation scripts, which have the suffix .preinst, but I think that a package is much more likely to create a new user in a postinst script.
Did you have a particular example in mind?