Browserprint: Browser fingerprint tool now can guess client OS even when spoofed
Guessing OS is pretty simple though, I recommend the book "Silence on the wire" [0] for a thorough explanation of passive network fingerprinting.TL;DR is that the each TCP stack has unique characteristics that are hard to spoof (you'd have to bypass the OS TCP stack and build your own that mimics another) and definitely out of reach for tools that run in sandboxed environments (like browser extensions)edit: Also, the author of that book, Michal Zalewski, made open source tool p0f [1] that implements some of those techniques to identify spoofed user agents.  [0]: https://www.amazon.com/gp/product/1593270461
  [1]: http://lcamtuf.coredump.cx/p0f3/