Is there any way to determine current action (create or edit) in Sonata\AdminBundle\Admin\Admin::configureFormFields()?

You can also do this:

protected function configureFormFields(FormMapper $formMapper) {
  if ($this->isCurrentRoute('create')) {
    // CREATE
  }
  else {
    // EDIT
  }
}

with:

if($this->getRequest()->get($this->getIdParameter()) == null){
   // create
} else {
   // edit
}