Simple: one thing I'm learning about is RFCs for TCP/IP. I can literally go test it. It's like saying, "How do you know it is right when it says 2+2=4"? Some knowledge when taught is self-correcting. Other things I'm studying, like, say, tensor calculus, I can immediately use and know I learned it correctly.
TCP/IP is a great example though of something you can get seemingly correct and then be subject to all kinds of failure modes in edge cases you didn’t handle correctly (fragmentation, silly windows, options changing header sizes, etc).
