When conducting an audit of a web app, identifying those technologies and the versions in use is necessary to conduct a thorough reconnaissance and correctly map the attack surface.
Those technologies can usually be identified from the different elements:
Credits at the bottom or corner of pages
Common files (e.g. robots.txt, sitemap.xml)
Stack traces and verbose error messages
Automated scanning tools can also help identify which technologies are used.
Wappalyzer is a browser extension that can detect the use of certain software including CMS