Glass is good to test on.
It will not interfere with the reaction (that's why the test is in glass as well).
That stuff what plates and wash basins (and bath tubs) are made of, you know the hard white stuff (damn, what is the english word ??) is good to test on.
Avoid metal, it will interfere. Avoid paper (especially if it has something printed on it).
So, glass is definitely OK and all those white surface in the bathroom are OK ;-)
(Can someone help me out with the word for that white bathroom stuff here ??)
Rgds
aj