This is akin to "where there's smoke there's fire" (on the level of gossip and hearsay and rumors) except of course for the fact that a war was based on this information, many people died and a country fell into ruin as a result...
There was procedure to make it not gossip -- UN mandated inspections. Iraq blatantly prevented them, and intentionally made it look like they are stockpiling WMDs. Good play on their part to trick US and UK.
Look up "UN Security Council Resolution 678 and Resolution 1441".
It was pretty clear, to both the government with access to the data, and also to a lot of average citizens. It was kind of obvious it was a pretext for a misguided retaliation for 9/11, a wounded Bush admin lashing out, trying to look strong.
The Iraq government prevented inspections in defiance of multiple UN resolutions for many years, during several admins, not just Bush admin. Note it was UN rules, not US. It just no country besides US wanted to enforce the UN resolutions.
Iraq gov did everything to make it look like there are: prevented any inspections, blatantly lied to UN etc.