There is a truism in logic:
absence of evidence is not evidence of absence.
You can't prove anything becuase you can't find evidence. You can only prove something based on the existance of evidence, not the lack of it. You can infer a conclusion, but you do so at great risk, for exactly the reaons that you put forward.
There was a large amount of evidence that Saddam DID have WMD. Based on that evidence everyone was sure he had them.
The fact that we didn't find any proves that we couldn't find them, or that maybe they weren't there WHEN we went in, but it's not proof that they weren't there a month before. Yet people keep insisting, that if we couldn't find them, that's absolute proof that he never had any, despite of all previous evidence.