To quickly view the your Joomla! template's module positions:

In Joomla 1.0.x from the public frontend:
add: &tp=1 to the url (ex: http://www.yoursite.com/index.php?option=com_frontpage&Itemid=1&tp=1)

In Joomla 1.5.x from the administrative backend:
go to Extensions > Template Manager > Click on the template name > Preview