To figure out what technologies a site is built on :
A- General Ways.
B- Reading HTTP Response Header.
C- Specific For SharePoint , Using Service.
- You could easy look at, URLs, and file extensions like (Aspx, php, etc.)
- View Html Source Code, to check Some special Keywords.
- Using Tools like, https://builtwith.com/
B- Reading HTTP response headers :
The most effective way to do this job , by reading the http response header of the web site using something like Fiddler , PostMan, etc.
You will get plenty of information about the web site ,
Example using PostMan :
You can see the SharePoint version 15 , that is mean SharePoint 2013,
SharePoint version list :
SP 2007: microsoftsharepointteamservices: 12
SP 2010: microsoftsharepointteamservices: 14
SP 2013: microsoftsharepointteamservices: 15
C- Specific For SharePoint , Using Service: