If you're looking for a quick-and-dirty approach, you can check the REFERER header.
If you really want to make sure that the form was fetched from your site though, you should generate a token each time the form is loaded and attach it to a session. A simple way to do this would be something like:
$_SESSION['formToken'] = sha1(microtime());
Then your form can have a hidden input:
<input type="hidden" name="token" value='<?=$_SESSION['formToken'];?>' />
and you can check that when deciding whether to process your form data.