How to identify traffic sources for someone else's site?

Question

Marketing

There is an application site and I need to find out if they use any ad network or where the bulk of users come from. Banal things, but the site is not mine and for me the search for an answer is as if I were fiddling with a black box.
The audience is purely English-speaking, I don’t know anything about attendance. Maybe there is a service for checking the site in advertising networks?

Answer 1

There are alexa.com and similarweb.com services. They give a superficial idea of ​​the scope of services, sources of their traffic and audience. They also have paid versions that provide more detailed information, but I have not used them yet.
Similarweb seemed to me pretty close to the truth - I compared several services through it, for which I have direct access to statistics.

Answer 2

More siliconrus.com/2014/12/startup-spy