You are right, there is no way to be absolutely certain, and I am not asking for that. What I would like, however, is to be beyond a reasonable doubt that a country poses a threat to the U.S. before we invade them and overthrow their government. It is, after all, the governmental equivalent of an execution. I personally wasn't anywhere near this regarding Iraq given what we (the public) were told. Afganistan and the Taliban were a different story.
But most of the American public were in support of the Iraq war, and that is what disturbs me. Despite no solid evidence that this foreign nation was a threat to us, a vast majority supported pre-emptively invading that country and deposing the government. America has never been like that. I probably am being too cynical when I say that fear is the root of it, though. It really is trust. Trust that, despite a lack of demonstrated evidence, their government would not mislead them into an unjustified war.
Regarding the intelligence that was used to provide the foundation for the war's justifications, I am going to have to consider it as being shaky until we actually uncover some hint of the hundreds (or was it thousands?) of tons of chemical and biological weapons that were claimed. You are perfectly welcome to view it as rock solid, if you wish.