Using input capture when the PWM duty cycle is 100%
Obviously you cannot measure the duty cycle of an 'always high' signal. You can however detect that there have been no transitions in the last period or two.
How you do this varies with what hardware you are using, and how many periods you can tolerate elapsing before you know about it.
One method is to use a retriggerable monostable external to the MCU. This could be a 555 or an HC123, or as crude as a diode/resistor/capacitor followed by a schmidt buffer. If the transitions on the signal stop, then eventually the monostable time will run out, and it can interrupt the MCU.
If the MCU has watchdog hardware, you might be able to use that directly, if you're not using it for its proper purpose, and it's fast enough.
Different MCUs can detect activity on pins in different ways, read your MCU pin description carefully.