How to debug Drupal Forms
You need just to implement the hook_form_alter in a test module this way:

function MYMODULE_form_alter(&amp;$form, $form_state, $form_id) {
  switch ($form_id) {
    case 'taxonomy_form_term':
      dpm($form);
      break;
  }
}


where $form_id is form identifier (in the example code I captured the new taxonomy term entry form) which you can find through firebug.