There are cases here where companies used GPL code without releasing their changes.

How do licenses of a source code check if the people using their code is complying with the license it uses?

https://www.reddit.com/r/embedded/comments/18gie6l/how_do_li...

The fastest way is often to just run the "Strings" program on the software. Often it will dump out a bunch of strings that match those in the Open Source project: Error Messages, Logging messages, etc. Sometimes if they're really sloppy it'll spit out the name of the GPL program/library directly and a version number.

I often add magic arrays to my code. So.. if I find them in a binary blob...

Have there been any lawsuits involving breach of open source licences?

https://opensource.stackexchange.com/questions/11452/have-th...

Suspecting users can try the software to see if it has the exact same functionality or bugs as the copied GPL library. This is of course not a definite proof, but some amount of rare enough coincidences can be considered as a very strong sign for copying. Legal measures can be taken on account of these evidences.

And of course there is always the option of a whistleblower.

Granted that means the 'smart' infringers are likely to slip through the sieve, but at that point they'll have to essentially be re-writing the code anyway, and lose most of the benefit that they'd get stealing the GPL code (they'd have to hand-roll any bug or security fixes back into their stolen-but-obscured GPL code)

Also I don't think it's that easy to conceal and not sure any serious company would risk the liability.