Inside you'd find an immense amount of mass at an extraordinarily high temperature. Your McGuffin device would measure those high temperatures and the gravitational force being exerted on it. It would see light (probably a blinding amount of it) if it looked at the surface of the black hole, and if it looked outside of it it would see the stars as well as any matter or light approaching the event horizon. All of that external stuff would slowly "fall" towards the surface but eventually appear to freeze right at the event horizon after being completely redshifted.
What do you think happens in black holes