The sensor opend the valve at 14.5psi. I don't know of a way to test it. At $56, it's not an item you should replace right away unless you KNOW it's bad.

http://catalog.eautopartscatalog.com/mercedesshop/wizard.jsp?partner=mercedesshop&year=1982&make=MB&model=300-TDT-001&category=B&part=Boost+Pressure+Sensor

It's my belief that ANY car/truck with a turbocharger should have a boost gauge of some kind. You should get one asap and find out exactly how much boost you have. If you keep an eye on it every now and then to make sure it does not go above 13psi, you should be fine.