I think, you're mistaken in how `if` works...?

    global $wp_query;
    $postid = $wp_query->post->ID;
    if ( ! empty( get_post_meta($postid, 'TourURL', true) ) ) echo '<P>Show Link</P>';
    else echo '<P>No Link</P>';
    wp_reset_query();

This should do what you want - if I understood you correctly.

**// Edit**

If you want to work with the meta data, you might want to use a variable to store the data, then check if this variable is empty.