Yup, there are some resources over the internet that tells you about the specific website that whether it is using WordPress or not?
Take a look at: How to Know a site is using WordPress or not
If you are really curious to know about the site you love the most, or you are in competition with it that which theme they are utilizing and the list of WordPress Plugins. This really happens when you are a beginner and you are inspired of a specific website, want to create site like that and want to spy on it.
So, here are some web resources that tell you about a specific website, Which WordPress theme it is Using? Which WordPress Plugins are used?
Just insert the URL of the site, you want to spy on.
Find out the technology behind websites you are spying. Provides you with the pro account used to create lists of websites from their database of 6000 web technologies and a quarter of a billion websites using shopping carts, web analytic etc.
This is website which tell the about a website in very details: theme name, theme homepage, description, version, author, author homepage, license, theme screenshot and the list of WordPress plugins used in that website.