How to know if finger is on joystick if it is at horizontal & vertical zero

I'm creating a jetpack (mobile) controller where left joystick is used to control forward and backward movements and right joystick is used to control rotation and upward movements. What I want is player to go upwards whenever user is touching the right joystick even if horizontal && vertical axes both return zero. So if there is a finger on the right joystick player goes up, similar to GetButton or GetKey(some keycode).