How can we know that a site is a Sharepoint Site?
Because its bloated and slow? Just kidding.
If you mean as a user, you can do a view source and there are several indicators, such as
<meta name="GENERATOR" content="Microsoft SharePoint" />
One way is to request URL /_vti_bin/owssvr.dll. If you get back a response other than 404 then you know it is a SharePoint site.
I explain some more details in this blog post.
I will try these to check if a site is a Sharepoint site
1) Check Url, if it has /sites/ in the url
2) check if accessing [ServerName]/_layouts/images/ERROR.GIF gives a Image
3) check if accessing [ServerName]/_vti_bin/sites.asmx?wsdl will display wsdl for the webservice
4) check if source has
<
meta name="GENERATOR" content="Microsoft SharePoint"
>
5) check if source show usage of init.js, core.js
6) check if _spBodyOnLoadWrapper function is being called on page load.