The test method depends on the unit you have. Typically they put out a very low voltage that is linier across the pressure range. If you get the spec. on it you can measure the voltage at a given pressure.
You can usually find the specs if you know the make/model of the sensor.
The system has two components, a sender (pressure sensor) and a receiver (gauge/read-out). You can test the read-out by feeding it a signal - either with a signal generator, or by putting a different known good sender of similar voltage range to it. For example, if your MAP sender operates in the same voltage range as your oil pressure sender, you can swap the leads to see what happens. The reading you get will be way off, but it will tell you if it is working.
If you have a Vans gauge set, I would suspect the gauge is the issue rather than the sender.