There are many free online tools available which let you know web technologies are being used.
Wappalyzer is a cross-platform utility that uncovers the technologies used on websites. It detects content management systems, ecommerce platforms, web frameworks, server software, analytics tools and many more.
BuiltWith is a website profiling, lead generation, competitive analysis and BI tool providing technology adoption and usage analytics.
W3Techs provides information about the usage of various types of technologies on the web.
SimilarTech technology provides partners with crucial insights into technologies from across millions of sites to leverage information and grow businesses. We offer analysis tools to spot trends, research markets, generate leads and prospects. Track and list an entire category of web technologies, follow real time traffic data and accurate international coverage.