On AWS, you can enable CUR (cost and usage reporting) and get detailed, line-item billing figures that you can audit.

And naturally, companies like Cloudability [now Apptio] and others have sprung up to do parts of this for you [at a fee, of course...]

https://docs.aws.amazon.com/cur/latest/userguide/what-is-cur...

I'm sure other cloud vendors have similar functionality (because they need this on the back end to do their own billing anyway).