How can a PLC know that the motor is turning or not

Explain how can a PLC determine if the motor is turning or not

You have several methods to do that

Firstly you have to give the PLC an input such as (auxiliary contact from contactor or VFD or proximity switch or encoder any other physical detector that applied a voltage to the PLC input when Motor has turning). Of course, you have to program the input to make the PLC sense the motor turning.

The shaft encoder is the best suited for sending pulses feedback to the PLC. You can use servo motors that have inbuilt shaft encoders or mount shaft encoder externally to be driven by motor shaft or machine main drive gear. Or cheaply one can make use of an inductive sensor to read motor shaft rotations and rely the feedback to PLC.

There are multiple ways that can tell the motor is running or not

Like Motor contactor contacts, tacho sensor feedbacks, current consumptions, so on and so forth it depends on what is your mode of operation of your motor is.