I can't say that I knew with 100% certainty that there were no WMDs in Iraq but I was highly highly highly skeptical. For me, you don't invade another country unless you are at least reasonably sure about it, which means evidence.
You make it sound like everyone believed that there were WMDs and some of us now are just claiming that we didn't after the fact. According to the Harris polls, the highest percentage of Americans who believed was 81%, immediately before the invasion. That was after weeks and months of propaganda aimed at us. And even then, that meant one in five Americans did not believe it.
Why didn't I believe the administration?
1) Sadam was not a religious zealot. In fact, he had been criticized by Muslim fundamentalists for not being religious enough. My assessment of him was that he was a two-bit bully dictator who cared more about preserving his own power and wealth than anything else. People who only care about their own power and wealth will oppress and even slaughter their own people; they may even invade a neighboring country to control their resources. But they do not stockpile weapons of mass destruction and aim them at superpowers and their allies. Only strong ideology causes someone to do something like that.
2) The accusation came out of left field. Shortly after 9/11, they identified bin Laden and Al Qaeda as the culprits, and at that time, they said that Iraq was not involved. And then all of the sudden it became Iraq has ties to Al Qaeda. Granted, they may have learned something to make them change their minds, but no evidence was ever presented to support this.
3) There was no frickin evidence presented. That was the bottom line for me. If you have evidence that leads to you believe that Saddam had WMDs then tell us what it is so that we will know that you're making the right decision and support you. Yes, I understand that some things can't be released for security reasons but at least give us hint. It boggled my mind that we were inching closer and closer to war, week by week, day by day, with out any real evidence presented to justify it, only accusations and rhetoric. I kept thinking that someone somewhere would say before it was too late, 'wait a minute. we're talking about WAR here, people dying, do you have evidence to back up your claims?'
I said in another thread that Powell's testimony in front of the UN did shake my resolve a little. But no way and no how did even Powell convince me that Iraq was a sufficient threat to warrant invasion.