Currently one cannot see who is the author of an announcement/alert besides looking in the database. By the following additions an user with manage permissions for announcements/alerts can easily check the author:
Add after https://github.com/liferay/liferay-portal/blob/master/portal-web/docroot/html/portlet/announcements/view_manage_entries.jspf#L59 :
row.addText(LanguageUtil.get(pageContext, entry.getUserName()), rowURL);
When managing announcements/alerts and choosing a scope, the portlet will show a list of all existing announcements/alerts and this list will show the author after adding these lines.