The "what is this strange filter and what lenses does it fit?" thread

[AgX]

To determine the distance at which a close-up accessory lens focuses a camera, measure the distance at which it focuses the sun or some other distant light.

You do not need the sun.
Set a SLR at infinity, attach the close-up lens, measure the focusing distance to the close-up lens.

Calculate the reciprocal of that distance in meter and you got the standard designation of such lens.