The 944 altitude sensor is really just a simple switch. At elevations above roughly 1,000 meters (~ 3,281 feet) it closes and connects its two terminals. When that happens, the DME leans the mixture by about 6% to account for the reduced oxygen in the air. So it's a pretty crude yes/no system for fuel correction at higher elevations.
If you never drive in the mountains and suspect the sensor is bad, you can simply unplug it. With the sensor unplugged, the DME will stay in the “low elevation” mode. Likewise, if you live somewhere like Denver and never drive below 1,000 meters, you can short the two wires going to the sensor. That tells the DME that the car is above 1,000 meters.
But if you actually need the sensor to work correctly, and you suspect it may be bad, here is a way to test it without driving up and down a mountain with the sensor and a multimeter in your lap.
Since the sensor responds to air pressure as a proxy for elevation, you can simulate higher elevation by applying a small vacuum to the sensor. You need about 3.4 inHg of vacuum at sea level (and proportionately less as your test elevation approaches 1,000 meters). Air pressure varies with weather and isn't an exact proxy for elevation, so you are looking for directionally correct results -- not exact numbers. Just thought this was kind of cool.