If you spot a GPL violation (someone used code under your copyright and did not offer source while distributing binaries), get in contact with the violator and ask to resolve the situation. Document the mails, data of when you contacted whom etc. .
If it's not your copyright but you spotted the violation, get in contact with the original copyright holder next to the violator and tell her/him about the violation as well. Advisably before you contact the violator.
If the software is a combined work with many copyright holders, the same applies to you if you're only one of the copyright holders or only a group of copyright holders (and not all). It's your copyright then, and the GPL applies to any kind of derivates - large or small.
If you're due to technical limitations unable to gain knowledge if a probable violation really is a violation, you need to look for other ways to find out more. E.g. by asking question to the probably violator, by getting access to binary or even sources through third parties etc..
If the code in question is under GPL v2 and there is a violation of the licensing terms, termination applies. The violator has lost all rights forever to make use of the GPL'ed code. He must get in contact with all copyright holders again and ask for reinstatement of rights, otherwise still using the code violates copyright.
If the code in question is under GPL v3 and there is a violation of the licensing terms, termination is in effect as well but the violator can recover from this by stopping the violation on it's own - unless a copyright holder asks for more.
Edit: This is merely general information. If you want to look into the concrete legal side of this you should be a copyright owner and you should contact your legal team upfront to find out what you need for a proof and what is suggested to solve a violation even w/o the law as this is much more practical.