Yes, use a transistor, see attached schematic. I assume you don't need to measure the amplitude of the signal, just see whether it is there - otherwise you can't use a digital pin. The diode D is only needed if the input signal is coupled through a capacitor. The transistor can be any small signal NPN type. The resistor values are not critical, I suggest 10K for both of them. You can omit R2 if you enable the internal pullup resistor for that pin.
