Prevent direct access to a web page except redirect
Add a Preparse field to the entry type you're posting to and make it generate a random string on entry save.
Then make the Guest Entries plugin redirect to a URL with that Preparse field's value in the query string.

&lt;input type="hidden" name="redirect" value="success?id={id}&amp;key={preparseField}"&gt;

You should now be able to get that entry model in your template and test against that Preparse field's value.

{% set query = craft.request.getQuery() %}
{% set entry = craft.entries.id(query.id).preparseField(query.preparseField).first() %}

{% if not entry %}
    {% exit 404 %}
{% endif %}