Actually, there are two bodies that are fully capable of determining compliance with international law. The first is the UN Security Council, which is charged with taking action on threats to international peace. The UNSC has, of course, avoided this subject.
There is also the International Court of Justice, also known as the World Court. It is an integral organ of the UN. This court ruled in 1986 that the US had violated international law by mining Managua’s harbors. The US has since refused to knoweldge that the Court has jurisdiction to hear such matters, so don’t hold your breath waiting for the ICJ to issue a verdict in this matter. Little more info.
There is the potential that there may in the coming years be a third body, the International Criminal Court. It will have jurisdiction to prosecute the crime of aggression, or aggressive war. That crime is currently undefined for purposes of the ICC, so really, it’s neither here nor there.
A significant number of intellectuals have weighed in on this as an academic matter, but the most authoritative opinion to date comes from the UN Secretary General, which was stated in a media interview, not as part of an official decree or anything: [Was the war in Iraq illegal?] "“Yes, if you wish. I have indicated it was not in conformity with the UN charter from our point of view, from the charter point of view, it was illegal.” Link.
And I think this side commentary about whether there is international law or not is really on the verge of being poorly informed. There has been a formal body of international law since the time of Grotius, so we’re talking about a concept of law that has been formalized for centuries. It has a variety of sources, leading to considerable complexity. Customary international law has developed through precident, and treaty based law is really a complex maze of paper that sometimes have enforcement mechanisms, but often do not. It is a different kind of law, just like how criminal law is not the same as tort law. Just because someone does not go to jail for being civilly liable for some act or another doesn’t mean that tort law does not exist; by the same token, sometimes the only “punishment” meted out by international law is that all other nations know that Country X doesn’t play by the rules. That’s unsatisfying to many, but it is what it is.
However, the idea that international law does not exist or does not have a corpus is just incorrect.