Detect if a language is translated or not
You could try adding something like the following to the end of your template. This is assuming you only have 2 locales.

{# test for the current entry locale (i.e. spanish) #}
{% if entry.locale = 'es' %}

    {# retrieve the same entry in the original locale #}
    {% set localizedEntry = craft.entries.id(entry.id).locale('en') %}

    {# compare the body content, and append additional content #}
    {% if localizedEntry[0].body == entry.body %}
        Sorry, not translated yet.
    {% endif %}

{% endif %}