config.xml

<?xml version="1.0"?>
<config>
  ...
  <global>
    <events>
      <controller_action_predispatch_routename_controller_method>
        <observers>
          <module>
            <class>module/observer</class>
            <method>checkCaptcha</method>
          </module>
        </observers>
      </controller_action_predispatch_routename_controller_method>
    </events>
  </global>
  <default>
    <captcha>
      <frontend>
        <areas>
          <contact_form>
            <label>Contact Form</label>
          </contact_form>
        </areas>
      </frontend>
    </captcha>
    <customer>
      <captcha>
        <always_for>
          <contact_form>1</contact_form>
        </always_for>
      </captcha>
    </customer>
  </default>
</config>