For others who'd stumble here, in modern browser you can see it in the LocalStorage. I use Chrome and it's in the dev toolbar under Resources.
In my case we also want to save the css file automatically (we have a tool that generates a template), we can do it easily with javascript.
This returns the generated CSS, just replace it with the right path, as you see it under the Resources tab:
localStorage.getItem('http://domain.com/css/main.less');
Then we send that through Ajax to save it in a css file. When switching to production we remove the less and replace it by the generated css file.