for angular 5 and below ,In your Chrome browser open the Angular site https://juliemr.github.io/protractor-demo/

press F12 then go to console and type angular

angular {element: ƒ, bootstrap: ƒ, copy: ƒ, extend: ƒ, equals: ƒ, …}

In angular site it will give output similar to above . For none angular it throws an error.

Uncaught ReferenceError: angular is not defined at :1:1

Check out the Chrome extension AngularJS Inspector. Great for developing in Angular and certainly gets you what you want.

https://chrome.google.com/webstore/detail/angularjs-inspector/gjhmfjbfdbeeekiijofbikifokdkfhcc

I use chrome dev tools (F12) to view all the .js files.
Go to the 'network' tab, filter only '.js' files and have a look at the preview of the content, you might see some clues.
For example, I saw in some sites a .js file of angular with comment of the angular version (1.5.8)

Another option:
Try this site: https://www.similartech.com
It has an option to enter a site URL and it will show what technologies it uses.