There are many such circuits available. To "sense" means to translate the high DC voltage to a lower level that can be processed easier. The straightforward way is to use a resistor divider, say 100 MOhms as the load, with a "tap" on it close to ground. What is your "sense" input?
With DC current the situation is similar, use a shunt resistor, say 0.1 Ohm, then across it you get 3V to process. Certain problem may be the voltage to ground on the current loop but there are opamps designed for this purpose.
Knowing Ohm's Law is enough, plus some knowledge of DC circuits.