Formidable's conditional logic works off of other fields on the form. If you know jQuery, this can easily be done with a small jQuery snippet. You're on the right track with the entry ID field. It doesn't exist on a new entry. It's not created until the entry is submitted. As you thought, it does exist on edits and is the value of a hidden field named "id". When you write your jQuery, save it in the before or after fields section on the form's Customize HTML page.

Just as a follow up.  I was thinking, that when you create a new entry the form is usually inserted into a  WP page.   There are no parameters in the URL  However when you edit an entry, the entry id is usualy contained in the URL, ie. somewhat like this: https://website.com/somepage/?entry=[id]     so I tried the [get param=entry] method as a default on a hidden field, and boom, it works.  if there is a value to entry it is an edit, if no value, it is a new entry.   anyway, an alternative that is quicker and easier for anyone who wants to determine whether its an edit or a new entry for conditional logic.
 
thanks again.  hope this heps.