Here's a very simple way to detect if a form has been submitted, I've recently wrote a plugin for dynamic forms.
The problem I had was being able to detect when a form was submitted, normally I would check for submit name array within the post array but since the plugin allows for full control over the form's elements including the submit button's name I had to do it a different way.
I could of had a hidden field in the form and then check for that but a much simpler way is letting the server detect the method request:
if($_SERVER['REQUEST_METHOD'] == 'POST'){
//submitted
}