You may have a couple of options
The first, as mentioned elsewhere, is to approach the authors of the open source software at issue, and get them to initiate an Intellectual Property suit. They are the only ones who can do this.
Beyond that, there seem to be a number of License Enforcement bodies who are acting on behalf of open source projects.
One example is the Software Freedom Conservancy. Their model implies that they act on behalf of member projects to defend their rights. So it would behoove you to determine if the project at issue resides under the roof of one of these organizations.
The FSF has a clear and detailed page listing the steps to take if you suspect a violation. If the copyleft license is one of theirs, and they own the copyright, they say they will take action.
I would have thought that the groups who create the licenses (FSF, Apache, OSI, etc) would have an interest in helping enforce them generally. It seems it should work a little like trademark law: if you don't protect your trademark, you lose it. In the case of the FSF, for example, if they allow GPL violations to stand simply because they don't hold the copyright on a particular piece of code, then they weaken their license.