You can handle this a number of ways.
1. Conditional
<% if @document.title?: %>
<%= @document.title %>
2. Postfix Conditional
<%= @document.title if @document.title? %>
3. Console
If you would like to receive a warning you can call console.log("whatever you want here.") and it will be output by node.js into your terminal.
<% if @document.title?: %>
<%= @document.title %>
<% else: %>
<% console.log("No document title in " + @document) %>
4. Query/Collection level
If you would like to not render documents without titles you could handle that at the query level in /docpad.coffee
pages: ->
@getCollection('documents').findAllLive({title: $exists: true}, [pageOrder:1,title:1])
This will check to make sure the document has a title before putting it into the pages collection.
5. Prototype the document object
You could rewrite the document object I suppose as well to overload the attribute calls and have it output a console warning in the event that that attribute doesn't exist, but that'd be much more low level...