How do I tell the document has changed?

I'm trying to splice a custom forms editor into elinks.  This editor 
will cache some of the information in the form for faster response and 
some intelligent editing functions.  This is for a custom front end on 
an embedded machine.

The editor is hooked into src/viewer/text/form.c::field_op.  I need to 
figure out a way to tell that the document has changed (i.e. the user 
has navigated to a new page) so the editor can flush its cache and parse 
the form again.

Is there something that is guaranteed to change on load/reload that I 
can check for quickly?  Perhaps something like document_view->name?

Thanks,

--Yan