DeLand RC Club flys ON airport property, they have a silhouette of an average 60 size model placed 400 feet from the flight line for reference.

They can petition the manager for a waiver to I think 1000 feet or 800, something like that, when they host an event, the airport grants it and puts out a NOTAM so the full scale guys know they are there.

Only other way is telemetry. I know my camera drone stops climbing between 395 and 400 AGL and I can barely see the thing, but gliders I've had over 1000, even at that, my 6m starts getting pretty small.