In version 3.3 new jQuery custom event triggers were introduced:
New: Introduce 5 new jQuery custom event triggers
- wpcf7:invalid
- wpcf7:spam
- wpcf7:mailsent
- wpcf7:mailfailed
- wpcf7:submit
You can use wpcf7:invalid like the example below:
$(".wpcf7").on('wpcf7:invalid', function(event){
// Your code here
});