Tire pressure

How on earth does the tire pressure indicator on the dashboard know the tires are low?

Either by the ABS system monitoring a sudden change in the rolling circumference of one tire compared to the others, or by a actual pressure sensors in the wheel.

Are you guessing one or the other or are both methods used?

Are you guessing one or the other or are both methods used?

Both methods are used depending on the make model and year.