How do I determine which side of my filter is coated?

The coated surface is easily determined by looking at the edge of the substrate, from the direction of the center of the filter at a slight angle so looking at the inside edge. If you can see the actual edge (thickness) of the glass, then the coating is on the other side. From the coated side, the edge is not visible. This is more difficult to check on coatings that transmit in the visible, but the edge can still be detected by viewing the filter at a steep angle.