If the ports are swapped then the sensor will read a large negative differential pressure. If this happens during calibration it will abort with an error. If this happens later on the airspeed will never be > 0 and AUTO FW flight will fail badly (diving for airspeed).
airspeed ~= sqrt(2 * p / rho) where p is the differential pressure and rho is the air density.
If you press against the pitot it will hold pressure, which is why you need to cover it without touching. However after calibration a nice quick test is to press your finger against the tip of the pitot and hold it. You should see the system read and hold a positive airspeed until you release.