I struggled with this today whilst trying to figure out Apache's config options.
On Ubuntu you can do the following:
$ dpkg --get-selections |grep apache
apache2 install
apache2-mpm-prefork install
apache2-prefork-dev install
apache2-utils install
apache2.2-bin install
apache2.2-common install
libapache2-mod-php5 install
libapache2-mod-wsgi install
This tells me that I have mpm-prefork.