ORIGINAL: Commander_Drake

For sailplanes I have seen people use reference cards that they hold at arms length. You match the picture on the card with the apparent size of your sailplane and it gives you an estimate of the height (or distance from you). I think they made these cards up themselves. I don't know if they can be purchased.

You could easily do it by setting the sailplane on the ground with its nose toward you and measure out a distance like 100 feet, then hold up an index card at arms length so that it looks to be just under the plane and mark the ends of the wingtips on the card and write 100 feet under it. Repeat for other distances.

Not too accurate but it's cheap.