Suggestions for backup motor control

I have a 5hp motor that drives a pump to circulate hot water from a boiler. Since the pump has to run 24/7, there is a backup motor and pump in case the primary one fails.

The problem is that they are not connected in any way--in other words, if the primary motor fails, the secondary must be switched on manually. I'd like to set this up so that if the primary fails, the secondary will start automatically. I have all the wiring figured out except how to detect if the primary motor is truly running or not. I will be using a programmable relay and can monitor the voltage from the breaker preceding each motor, so I will know if the breaker is tripped. However, the motors have built-in thermal overloads which wouldn't trip the breakers. (They haven't stopped due to heat as of yet but I want to cover all the bases.)

Now I'm thinking of putting some type of current sensor on the primary motor's input power. Is this the right approach for this application, or is there a better way? I haven't used this type of device before and would just need a digital output or relay to interface with the PLR. And I'd like to do it for under $100 if possible. BTW the motors are 208vac single phase but 110v is also available.

Thanks for everyone's help!

I would use a pressure switch or flow switch to monitor your process.
If pump should be on and no pressure then turn on secondary.
Or if pump should be on and no flow then turn on secondary.

I agree with Mickey. You didn't mention how the motors and pumps are connected, but if there is a coupling, it would be possible for the coupling to fail, and the pump stop turning, but the motor is still running. (I've been been bitten by this before). It's best to monitor the process, or perhaps verify that the pump shaft is turning.