Keeping track is what they call orientation. And it more or less comes down to trusting your fingers that the aircraft is going to do what you tell it to do. If I lose orientation in a flight I trust that I had the nose facing away from me so if I spin it 180 degrees with the controls then it will come back to me. So basically lots of practice.
There are really advanced telemetry systems in the bigger birds. They tell you vibration levels, head speed, amperage draw, etc. Just like a real aircraft. It will also alarm if each value reaches a certain threshold. And this stuff is surprisingly (and relatively) cheap. Most of the modern RC transmitters come with the ability out of the box.
For the smaller aircrafts (like mine) you will generally know if something is off. It won't react right, it will wobble in flight, drift. Things like that.