Advanced telescopes and radars detect and track celestial bodies and determine their trajectory. We pretty much perfectly mapped the gravitational effects of all the large bodies in our solar system so a computer calculates future trajectory with the data observatories gather. Calculations are relatively simple even a phone app can do it with right data inputs.

But problem is the ones we cant see. Radar is useful in close range. And optical telescopes require astroids to pass in front of a light source ie sun