i think all stake holders involved will keep track.
Sounds logical, but i haven't seen any individual(again to the best of my knowledge) doing a solo defense of his GPL'd code.
If a competitor of X
is violating GPL, X would mostly definitely be on to him! Perhaps point it out and inititate action through FSF.
That's generally the way it has been. Either the incident has been reported to FSF(anonymously or otherwise) and FSF alone or FSF and the entity therein have taken the violator to book. An interesting point however is that the first contact is made discretely to the violator informing of the violation, then helping out with correcting the situation. If however the entity refuses to co-operate then matter goes into arbitration or eventually litigation.
Regards,
- vihan