Very interesting discussion but please allow me to ask a stupid question.

How does any of us truly know if we are above 400 feet?

I fly 40 and 60 size planes at what I consider one mistake high. While I am confident that my current flying situation keeps me below 400 feet what if I start flying larger models? Short of telemetry I don't believe I would be able to tell if I'm exceeding or staying below 400 feet with any certainty.

Thanks to anyone willing to share their insight on the subject.