The Fast Way
Paste the URL into a tech stack checker. It returns the CMS or website builder, active theme, hosting provider, CDN, and analytics platforms in a single report, with no technical knowledge needed to read the result and no login required to run the check. This covers the question the vast majority of people asking this actually have.
For checking your own site, a competitor, or a site you're curious about, this is the complete answer for most purposes — there's rarely a need to go further than the automated result.
When You'd Want the Deeper Explanation Instead
The quick lookup answers what technology a site uses. It doesn't explain why or how that detection actually works under the hood — which HTTP headers matter, how HTML source signals differ from DNS signals, or how to manually verify a result. For that level of detail — useful for developers, technical SEOs, or anyone building their own detection logic — the full methodology guide breaks down all three detection methods individually.