How do I access the "#default_value" property from $form_state['values']
The array indexes contained in $form_state are reported in Form generation.

The default values are only present in the $form array, which is passed as argument to the form validation handlers, the form submission handlers, and any implementation of hook_form_alter() (including hook_form_FORM_ID_alter()).

The only way to access the default value for a form field is to know the array structure of $form. If the code is using a form created from a module you are writing, that is not difficult; if the code is using a form created by a module you didn't write, you need to check the structure of the form array created by that module.