How to know if a signature has been updated

How is it possible to check if the user has updated a signature in form validate?
If you edit a form with a signature field, you can sign and submit the form or you can just submit the form without changing the signature.
In the form validation, I have tried a check like the following to see if the signature has been updated:

function is_sign_updated($form,$form_state){
	$old=$form['my_sign_field'][LANGUAGE_NONE][0]['data']['#default_value'];
	$new=$form_state['values']['my_sign_field'][LANGUAGE_NONE][0]['data'];
	return !($old==$new);
}

But it seems that even if you don't change the field value, a new png file is created each time you submit the form... thus this function will return true in all cases...

Any idea?