The counter argument is "you didn't personally observe it, thus you can't expertly speak to this point."

I'd charge Ken Ham to plant a tree in his backyard. Ten years later, he should cut it down and count the rings. There should be enough rings to account for ten years. Then it doesn't matter if you personally observed it or not, you can extrapolate based on the fact that all trees make rings in a similar fashion. 

I want Bill to destroy this observation argument like this. Please, please, please!