This is sky crane:
Mars has 1/3 of Earth's gravity, so how was sky crane tested on Earth? Did they even test it at all? Did they repeatedly do the math, and make sure everything works so that they are 100% sure it would operate without testing it?
As per this stack exchange answer, sky crane uses Hydrazine Monopropellant thrusters which is capable of 400 - 1300 N of thrust. How did they test these engines?
To be clear, I don't expect a video or footage of the sky crane being tested, but any official NASA or JPL article that touches on this topic would be sufficient.