The form submission endpoint doesn't give you any information if the submission is successful; whether or not the contact exists doesn't affect the response. The response from the form submission endpoint can be any of the following:
204 when the form submissions is successful
302 when the form submissions is successful and a redirectUrl is included or set in the form settings.
404 when the Form GUID is not found for the provided Portal ID
I'm not sure I'm clear on the issue you're describing; a 204 No Content is the expected success response for this endpoint; if you receive a 204, you can assume that the form submission was accepted. What is the data that you need?