External trigger pulse
Since you're only checking if a signal of a specified frequency range is being received, and not its width/duty (as per your comment), you can setup the (E)CCP module to capture some rising (or falling) edges to get the period of your signal. From that you can calculate the signal frequency and act accordingly. 

You can easily discriminate between a valid signal and DC because dc would simply not have a period while random noise can be filtered out.

If the ECCP module is not available (or it's not suitable to capture your frequencies, I don't honestly remember right now its usable range) you can setup a timer to emulate it in software by reading how much time passes between two or more rising or falling edges.