Retrieve post_type querystring value?

I need to retrieve the post_type querystring value so that I can display the appropriate page heading on my archive page based on content type. I’ve confirmed the querystring values exist using this code from the forum:

<?php
echo "<pre>"; print_r($wp_query->query_vars); echo "</pre>";
?>

http://wordpress.org/support/topic/how-to-get-query-string

However, when I use $_GET to grab the querystring value like this:

$post_type = $_GET[post_type];

I get no value. What is the secret to capturing WordPress querystring values for this kind of task?

Thanks!