How can I tell whether a (Drupal 7) web app was built using Angular by looking at the page source, and not having to ask the developers?
window.getAllAngularRootElements() worked for me
If it's an online site you can use http://builtwith.com/ and it usually can give you a good and useful bunch of information about that site.
However if they are using angular you can take a look at their page sources to see if they are using any attributes of angular like ng-repeat for example
You can also take a look at the sources that your browser get while browsing that site to see if javascript files for angularjs are included among the sources.
Install chrome extension React-Detector, it works!
Try
https://builtwith.com/
This is the website that currently gives you the most detailed information about what technologies a site uses. They will let you do 5 free lookups a day.
https://wappalyzer.com/
is a good one too, and serve as a complement to builtwith. I don't think there is a limit of lookups, but the results are less detailed.
Those two used together may give you more insight.