Is there a template tag (or any other trick) I can use to display all the variables available in a page?
There are several options (some of them already listed before):
django builtin debug tag - I use this boilerplate to display it properly:
{% filter force_escape %} {% debug %} {% endfilter %}
Use django template debug application's tags like attributes, variables and details or you can even set a breakpoint inside of a template where you can inspect everything with pdb debugger (or ipdb)
Django debug toolbar - has a template panel for this purpose
Usually all debug features work only when the app is in DEBUG mode.