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.
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.