how to get the form id of a particular form using Devel execute PHP in drupal 7?
I don't think it's possible to do this without something (either your custom module or another module) implementing hook_form_alter(). The Devel execute PHP form is in a block which has no real idea what forms are available on the page.

I'm sure you know the way to get the form ID but just in case this will print it at the top of every form:

function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if (user_access('whatever permission you want')) {
    $form['mymodule_form_id'] = array(
      '#markup' =&gt; '&lt;p&gt;Form ID: ' . $form_id . '&lt;/p&gt;',
      '#weight' =&gt; -10000
    );
  }
}


If you really don't want to do this, though, why not just use developer tools/firebug to inspect the form element on the page itself...you'll find the form id in one of the hidden input fields.