You can do this with Javascript by listening to the “change” event. jQuery also provides a method, like:
$('.field-class').change(function(e){
//Do stuff on field change
});
Note that this event provides a basic change event for input fields and you need more for other types of fields, or if you want your button to show immediately when someone starts typing. You can check out this thread on Stackoverflow: http://stackoverflow.com/questions/6458840/on-input-change-event
