detecting signal coming from another motion sensor that triggers camera's shutter
Sorry I am typing this here coz I don't have enough rep to comment. Anywho, most of the motion sensors involve PIR tech, so the best way is to connect your PIR Signal pin from your sensor to any analog pin of your arduino and test the kind of signal you're getting when the environment is idle and when movement is detected, Keep in mind of the region that the sensor covers too (ROI). That will tell you some things,

If the sensor works or not.
If the sensor value is analog or digital
If the sensor is active high or low
Trial and error on the actual ROI

After all that you can write your code around it.