If I configure a workflow to start when an item is changed, how can I tell which field changed to initiate the workflow? What I have in mind is I really only want the workflow to fire if one particular field changes and fall through any other time.
Your title is a bit different than your question so i'll answer what I think is your question with a tutorial article.
http://sharepointsolutions.blogspot.com/2007/10/sharepoint-designer-workflows-how-to.html