curl --head http://www.zappos.com/

It's worth doing yourself because they give different messages with each request in fields like X-Core-Value but here's an example for the lazy:

    HTTP/1.1 200 OK
    Server: nginx/1.1.14
    Content-Type: text/html; charset=utf-8
    X-Powered-By: Ponies!
    X-Varnish-TTL: 60m
    X-Varnish: 1001887650 1001881441
    X-Cache-Hits: 2832
    X-Varnish-Host: varnish04.zappos.net
    X-Varnish-ID: drupal
    X-Core-Value: 4. Be Adventurous, Creative, and Open-Minded
    X-Recruiting: If you're reading this, maybe you should be working at Zappos instead.  Check out jobs.zappos.com
    X-UUID: c687de54-4df3-11e1-b455-00215e22da70
    Cache-Control: max-age=583
    Date: Fri, 03 Feb 2012 00:05:21 GMT
    Connection: keep-alive