You can use the following example to check to see if a value has changed. In this specific example, if the value has not changed, an email notification will not be sent.

add_filter( 'frm_pre_update_entry', 'check_if_value_changed', 10, 2 );
function check_if_value_changed( $values, $entry_id ) {
  $field_id = 125; // change 125 to the id of the field to check
  $previous_value = FrmEntryMeta::get_entry_meta_by_field($entry_id, $field_id);
  if ( isset( $values['item_meta'][ $field_id ] ) && $values['item_meta'][ $field_id ] == $previous_value ) {
    // do something if the value did not change
	add_action( 'frm_trigger_email_action', 'frm_stop_email', 1 );
  }
  return $values;
}

function frm_stop_email(){
  remove_action( 'frm_trigger_email_action', 'FrmNotification::trigger_email', 10 );
}