David Keith provided these images of his coupler height gauge.  He made it by stacking layers of styrene to the under car body height (to measure the under body height on one end) and then adding a couple more layers and mounting a coupler.

The Sn2 coupler height standards are not the same as HOn3.  See standards elsewhere on this site.  Tom Troughton (sp?) made a similar gauge for Sn3.  His is a bit more refined than mine.

On the left is a tab for sliding under cars to verify that the body of a car is at the proper height. At right, a KD-714 HOn3 coupler has been added.

A coupler height check. This gauge does not yet have anything to center the gauge on the rails, but that should be easy enough to add.

Click buy a coupler height gauge if you do not make one.