In Windows 10 and many other proprietary operating systems, you can "seemingly" opt out of sending usage data. However, if the operating system is proprietary, how can we be sure that the OS is not continuing to send usage data? Is the source code examined by legal authorities?
No, there is no central authority that certifies such things. But if you're curious you can examine all of the Internet traffic and see if any goes someplace you don't expect.
By the way, even with open source it isn't as easy as just reviewing the source code. You have to build the operating system yourself, somehow verify the toolchain used to build the operating system hasn't been compromised in order to insert privacy bypasses (no easy feat) and even then there could be subtle things included in the source code in order to defeat source code analysis that could route traffic in non-obvious ways.