You can use domaintools.com to lookup the server information for a website and narrow down to whether it's open source / Microsoft:
http://whois.domaintools.com/stackoverflow.com
And after that it's a matter of looking in the footer for tip-offs such as "Powered by WordPress" or "vBulletin" etc.
![[愿得一人]](https://www.e-learn.cn/qa/data/avatar/000/00/00/small_000000012.jpg)