There are two parts to this. On one side, you have the "please follow the GPL if you're using GPL code" -- which is really just asking someone to honor a contract, more or less.
Then you have people like RMS, who believe that there should not be such a thing as proprietary software. They don't care if you aren't using the GPL -- no software should be proprietary, period.
It's may be possible to tell if there is GPL code included whether or not the source is reveal. A simple example is that some plain-text strings are visible in compiled binaries. In this way, poking around with a hexeditor may reveal strings which indicate part of the code is GPL.
Hmmm. Thankfully I only develop small services and haven't done any big monolith that people wanna audit. When I get to that point I'll prolly get sued and then try to figure out what to do while profusely apologizing and denying at the same time.