We do a lot of component caching and not all of them are updated at the same time. So we set host and timestamp values in a universally included context processor. At the top of each template fragment we stick in:
<!-- component_name {{host}} {{timestamp}} -->
The component_name just makes it easy to do a View Source and search for that string.
All of our views that are object-detail pages define a context variable "page_object" and we have this at the top of the base.html template master:
<!-- {{page_object.class_id}} @ {{timestamp}} -->
class_id() is a method from a super class used by all of our primary content classes. It is just:
def class_id(self):
"%s.%s.%s" % (self.__class__._meta.app_label,
self.__class__.__name__, self.id)
If you load a page and any of the timestamps are more than few seconds old, it's a pretty good bet that the component was cached.