On the topic of Seers, not only are they fairly rare (At the very most a tenth of the misting population), but Atium itself is very difficult to come by. You have to buy it off the Lord Ruler for ridiculous amounts of money, and I don't see many people going through all that to get a bead they'll just throw away testing.
Better idea: Copperclouds, somebody would notice because they don't have to actively try to find it out. It could simply be an accidental find.
With Atium, how many times do you accidently eat an Atium bead?
God says copper's perfect, but you might accidently learn otherwise. But God says only Mistborn get Atium, so why would any of th nobles waste time and money proving him right or wrong?