I don't know much about this but according to me if it is a standard radio control gear then it will be a standard servo signal. you won't be able to accurately measure the voltage with a multi-meter but it'll probably be between 0.3V and 0.7V less than the positive.
Receivers usually need 5V to operate.
If connecting positive to 5V then use a voltage divider or logic level converter to drop the signal down to 3V3 before connecting to your GPIO.