Quick and dirty method:

 

Append "_vti_bin/owssvr.dll" after the site:

 

Example: 

http://www.marines.mil/Pages/_vti_bin/owssvr.dll 

 

This should return your standard SharePoint page if the site is indeed running on SharePoint. 

Most folks don't go through the trouble of redirecting the old Vermeer shadow directories and/or disabling owssvr.dll (which would generally be considered bad for SharePoint..)  You can get some info on the old Front Page extensions remnant here: http://www.synergyonline.com/blog/blog-moss/Lists/Posts/Post.aspx?ID=24 

 

Stuff like this made SPS not-so-user-friendly back in 2001, but it is my trick of the day for confirming if a public website is running SharePoint.

( I also like when web admins on .NET boxes leave tracing on and you can use Trace.axd...  but that's me.. )  

 

-E-