Use callbacks.fired() to determine if the callbacks in a list have been called at least once:

// A sample logging function to be added to a callbacks list
var foo = function( value ) {
  console.log( "foo:" + value );
};
 
var callbacks = $.Callbacks();
 
// Add the function "foo" to the list
callbacks.add( foo );
 
// Fire the items on the list
callbacks.fire( "hello" ); // Outputs: "foo: hello"
callbacks.fire( "world" ); // Outputs: "foo: world"
 
// Test to establish if the callbacks have been called
console.log( callbacks.fired() );