A simple way to detect an AC 3 phase induction motor ON/OFF state
For most purposes, it is probably best to use a signal from the variable frequency drive (VFD or "inverter"). Most VFDs have analog signals, discrete signals or relay contact indications for external use. An output from the PLC would also be a good alternative.

You can probably get a variety of indications including: motor running, motor running above X speed, motor torque and others. If the VFD has a Modbus connection, it is probably capable of providing the best indication of most motor operational conditions while the motor is under the control of the VFD. Many of those indications should be available separately from the PLC if you want them that way. A VFD with Modbus likely has some customer configurable discrete and analog I/O points built in for external use. Study the VFD manual to se what is available and don't hesitate to use features intended for customer use. You should not need to buy anything else unless you need to know what is happening while the motor is not under the control of the VFD.