Is there any hook_node_pre_submit() like hook?
I think hook_form_alter is also a good option to overcome this issue. You can validate by altering the webform.

for example:

function mymodule_form_alter($form, &amp;$form_state,$form_id) {
  //dpm($form);
  if ($form_id == 'your_form_id') {

    //validate function call
  }
}