use SymfonyComponentValidatorConstraints as Assert;
use SymfonyComponentValidatorExecutionContextInterface;

//...

public function buildForm(FormBuilderInterface $builder, array $options) {
    parent::buildForm($builder, $options);
    $builder->add('enabled', 'checkbox', array(
        'required' => false,
        'constraints' => array(
            new AssertCallback(array(
                'callback' => array(
                    $this,
                    'validateisReady'),
                'groups' => $this->validationGroups))
        ),
    ))
    ;
}

public static function validateisReady($object, ExecutionContextInterface $context) {

    //..
    if($object->getItems()->count() < 1){
        $context->addViolationAt('enabled', 'items.missing');
    }
    //..
}