The scenario is this. Rented a PA-28 from my club and set off with a mate for a 200m x-country. Upon arrival, was told that someone was on the phone waiting to talk to me. I was somewhat surprised and impressed, although soon told that commercial traffic had been diverted because I was flying at 20,000ft. Hhhmmm, I explained I was squawking charlie in a PA-28 and that they should know that I'm a PA-28 and obviously incapable of reaching 20,000ft. Response was akin to beep-beep-silence-beeeeepppp. To the crux of the question then, how does one check for faulty trx altitude reporting ?!