Speed Density System Description
The speed density system is only needed when there is a mass air flow (MAF) sensor malfunction.

If the PCM detects a malfunction with the MAF sensor circuit, the PCM will default to speed density fuel management.

Three sensors provide the PCM with the basic information for the fuel management portion of its operation. That is, 3 specific signals to the PCM establish the engine speed and air density factors.
The engine speed signal comes from the ignition system. Air density is derived from IAT and MAP sensor inputs.
The IAT sensor measures the air temperature that is entering the engine. The IAT signal works in conjunction with the MAP sensor to determine air density. As the intake manifold pressure increases, the air density in the intake manifold also increases and additional fuel is required. This information from the IAT and MAP sensors is used by the PCM to control injector pulse width.

When/how does the PCM detect a MAF sensor failure?

Keith

The PCM compares the MAF output to what they calculated for VE table, if the values are not close then PCM assumes something wrong with MAF and switches to VE tables

If people do not muck around with porting MAF or TB then VE tables are only used for startup and fallback conditions.
Once people screw with the airflow path and/or MAF and VE tables then more chance values do not match forcing PCM to switch back/forth to VE tables.