How to monitor the correct rendering of JS widgets on the site?

Question

JavaScript

There was a task to monitor the fact of rendering widgets of third-party services. Subscription forms, payment widgets, things like Yandex.Posters - that is, JS is loaded entirely from someone else's server, but rendered on the client in the browser?
Let's say you can hang JS events on your objects and log them, on high traffic you can track the triggering of goals in metrics. And how to check that external services are rendered and triggered? Let's say the traffic is small, but very expensive. Suppose an event happens once a day, but its value is incredibly high.

Answer 1

If the scripts are loaded directly on the page - and work with your dom, then you can do everything the same as for your objects. If in an iframe, then in the general case there is little that can be done if external scripts themselves somehow do not tell you about it.
If you need to do this in a test plan in an environment you control, there are tools that take a screenshot and compare with what it should be, maybe something will work out with them.