Sure, just use LWP::UserAgent to submit the search terms "SSI +site:togetherhost.com" to google, then use the same script to pull the page www.togetherhost.com/hostplans.html

In the ancient and sage words of the philosopher Neil Gaiman -- "Google is your friend. Google loves you. Google wants you to be happy".

Less sarcasticly, although individual webservers may broadcast that information, it is generally not going to be available by querying the server, even if it was you can't assume that because the hosting company's server does/dosn't have SSI enabled the same will be true for customer sites they host.