A method of testing a test wafer includes shielding test centers on a test wafer using shielding tabs during the deposition of a layer. The test wafer has the same size and shape of product wafers. The shielding tabs are then removed from the test wafer. A plurality of predetermined points which are separated from each test center by a critical interval are checked, and whether each point is covered by the layer is determined through an interferometer or a microprobe. The test wafer is processed after adjustments to or maintenance on equipment, or after a fixed number of product wafers have been processed.