It's not easy to find the correct side of the lens because it retains a round plate shape even if it's flipped out. However, you can definitely find a difference in the edge of the lens. The lens is originally shaped like the edge is curled into a circle, but when it is flipped out, the edge is relatively outward.
These changes are, in fact, not as clear as shown in Picture A. If you look at the picture B, because the right side (Wrong side) also retains some shape, you may think it is not flipped out. But compared to the left side (Correct side), we can say the right side is flipped out.
Therefore, the user must flip it several times to find the correct side before wearing it.