We have lot of websites in internet and many of them are in SharePoint. So if you see any good looking site on internet and you want to check whether it is sharepoint based site or not?
There are some really cool exceptional ways.
Take "View Source" from any browser of your site. You will see something like <head> <meta name = "Generator" content="Microsoft SharePoint"/>
You will see reference JS from src="/_layouts/1033/init.js"
If its SharePoint site there are also webservice with it. You can call any of webservice (If banned you will see "Unknown Error" page). e.g http://SharePointSite.Microsoft.com/_layouts/Lists.asmx .
Otherway is check images folder available in 12 hive folder(e.g. http://SharepointSite.Microsoft.com/_layouts/images/accesssetting.gif), it will show you image available in 12 hive folder.