>>53727157
Exactly the same as any other software licence: it depends on where you are and how much you can pay for a lawyer.

>>53727157
GPL enforcement is why custom router firmware is a thing. The WRT54 routers used GPL code, and they got a judgement against them saying they had to publish the source under GPL because of it.

>>53727555
Sorry, let me clarify: How can someone be sure they're using GPL code if source is closed?

>>53727592
You'd have to prove that they're using some GPL code. Maybe there are hardcoded strings in the compiled program that can be traced back to open source code

>>53727592
They have to make the relevant source code publically available.
They don't have to shit with any proprietary code.

>>53727157
http://gpl-violations.org/about/

>>53727931
All additions and modifications to GPL code must be licensed GPL as well.

>>53727931
you need to read GPLv3. this is a serious point of contention among industry developers. It's why most companies use MIT or Apache licensing.

As for OP, short of reverse engineering like >>53727630 describes, you can't know. This is one issue that makes open source licenses problematic, but it's an issue with disclosure of anything in general. In the criminal justice system if the prosecution doesn't disclose certain information, the punishment is severe (the defendant could get a mistrial). There are similarly pretty serious punishments for violating copyright agreements. More serious than might seem intuitive.