Use callbacks.disabled() to determine if the callbacks list has been disabled:

// A sample logging function to be added to a callbacks list
var foo = function( value ) {
  console.log( "foo:" + value );
};
 
var callbacks = $.Callbacks();
 
// Add the logging function to the callback list
callbacks.add( foo );
 
// Fire the items on the list, passing an argument
callbacks.fire( "hello" );
// Outputs "foo: hello"
 
// Disable the callbacks list
callbacks.disable();
 
// Test the disabled state of the list
console.log ( callbacks.disabled() );
// Outputs: true