Installed Boost module. How do you tell it is working on your site?
View the source code of the page as you're browsing as an anonymous user. Boost inserts some metadata about the cached page ("Page cached by...") as an HTML comment:

  // Add note to bottom of content if possible.
  if ($_boost['matched_header_info']['comment_start'] &amp;&amp; $_boost['matched_header_info']['comment_end']) {
    $expire = $_boost['matched_header_info']['lifetime_max'];
    $cached_at = date('Y-m-d H:i:s', REQUEST_TIME);
    $expires_at = date('Y-m-d H:i:s', REQUEST_TIME + $expire);
    $note = "\n" . $_boost['matched_header_info']['comment_start'] . 'Page cached by Boost @ ' . $cached_at . ', expires @ ' . $expires_at . ', lifetime ' . format_interval($expire) . $_boost['matched_header_info']['comment_end'];
    $data .= $note;
  }