To test if the site is down, have a look at:
http://host-tracker.com
The website will be checked (mostly with HEAD method) by more than 30 servers, locating in cities all over the world. It will give you an overall availibility status of the site.
Also, you can monitor your own site too by free registration, which I find very useful.
This is status of the "other site" a couple of minutes ago:
http://host-tracker.com/check_res_ajx/1380122-0/share/