This device allows you to detect the presence of pure Vis in thaumic devices. Simply clicking on any type of thaumic device or conduit will show you how much pure vis it containts.
While the Vis Detector is in your inventory or hotbar it will also display a bar in the lower right-hand corner of your screen. This indicates how much Aura there is in your current chunk. The sphere below the bar will glow if the aura has the potential to increase due to the presence of unharvested Vis Crystals or similar things.