One way to monitor the business end of a RC device is to tap into all of the servo outputs on the receiver and decode their respective PWM signals. A much simpler interface, however, both in terms of wiring as well as code, is to find the multiplexed PPM signal and digest the values for all of the channels from a single feed.

Jordi from DIY Drones has a nice tutorial and demo video which shows you how to intercept the raw PPM signal with an Arduino and just about any common Futaba receiver (and probably many others). His demonstration also shows how to probe an unfamiliar receiver so that you can discover the pin that’s outputting PPM.

How to hack the PPM signal from any receiver (Futaba) with Arduino