You need to conduct a series of positive tests. As already mentioned, LWP (and it's derivatives) are what you want to use to write your tests. You will need to write a series of tests if you want to determine whether the whole of a website is up and running (rather than just a simple page).

I have previously used Test::Simple to build a set of tests that excercise various parts of a website. Example tests include:

• Get the homepage - as the user would see it. This tests a load balancer if present.
• Get pages from servers directly (without a load balancer)
• Login and retrieve the appropriate session cookies.
• Get pages that are only returned for logged in users.
• Submit a search to the search engine and verify that an acceptable number of results were returned.

Typically you try and test individual pieces of functionality so that you can pin-point the failure. This is especially important where a website relies on multiple hardware and software components performing together.