One way to test vision in order to detect even small changes when they first appear is to use the Amsler grid.

Follow these instructions:

1. Wear your reading glasses and hold the Amsler grid at a normal reading distance.

2. Cover one eye.

3. Look at the center dot and keep looking at it at all times.

4. While looking directly at the center, notice whether all the lines are straight and all the small squares are the same size.

5. If you should notice any changes on the grid such as distortion, blur, discoloration, or other abnormality, and it stays that way for a few days, see your eye doctor promptly.

6. Test the other eye in the same manner.

Also, it is helpful to monitor your reading vision.  Reading newsprint may become difficult even before there is a change on the Amsler grid.