You need a calibrated altimeter and a "decoder" to read the output of the encoder. In my case, I have a transponder with a digital readout, so it was pretty easy to adjust the encoder to correspond to what the altimeter said.
Otherwise, you will indeed need to pay someone to do it.
What I've been told is all installed transponders capable of transmitting (unless fuse is pulled and properly flagged as "Inoperable" ) must be certified every two years by an FAA approved avionics repair or calibration station. Repair stations can calibrate the encoder, calibration stations are not suppose to, but often do. (I believe these are the correct terms.)
This, IMHO is one of the most neglected certifications in any airplane, experimental or certified. A good number of airplanes are out of certification on their transponders, or never have had them certified at all. I found out the hard way ferrying a RV-9a from Washington St. The transponder was reporting 31,000'! Needless to say I got a phone call when I got on the ground, and got an education on transponder certification and the requirements there of. All Minn. Center wanted me to do is turn it off! Needless to say I complied.
...at a licensed shop. The first thing he did was set the altimeter to 29.92hg, then placed his receiver box in front of my airplane to check the reading.
