To set the angle of attack sensor to a certain angle we use a AOA rose.
There are different types of AOA sensors and different types of sensors. Usually we hook one up to both sides of the aircraft to simulate a certain aircraft nose up or nose down setting.

On this type we start with screwing the rose mount onto the fuselage.

Then we connect the rose onto the mounting block.

Now we can set the AOA sensor accurately to a certain setting. Here's 20 degrees nose down.

And here's 20 degrees nose up.

And here is 0 degrees. (level flight).

Now that we know that the sensor is physically placed in 0 degrees we can see if that is also what the sensor is sensing. In our case we have a SMYD (Stall Management Yaw Damper) computer.