header.php template: knowing what template is used

if you look into the code of ‘body_class()’ template tag
http://phpxref.ftwr.co.uk/wordpress/wp-includes/post-template.php.source.html#l352
there seems to be no single, simple command to show which template file is used.

you probably need to copy and use part of this code to get the information you want.

depending on the purpose of this knowlegde, maybe using ‘body_class()’ would do it for you?