\tracingall turns on all tracing. Put someplace near when you think the problem happens.
There are a lot of helpers which make things visible in the PDF. Mostly they have a form of \show... commands.
\replacemissingcharacters will replace characters missing in the font(s) with squares respecting font switches (\it, \bf). Underneath the squares are wanted characters, so though invisible, one can still search (in searchable PDF editors) or copy them to the text editor to see the missing characters.
Paragraph and page composition
\showmakeup shows how TeX composes text to paragraphs and paragraphs into pages. It inserts PDF layers in the output, which can be switched on/off. By default there are glue, hbox, kern, penalty and vbox layers. See \showmakeup to see other options.