When you pluck a pinch harmonic (an artificial harmonic in classical terms) on different parts of the string, you'll get a different pitch.
Through basic knowledge of theory. Know which note it is you're fretting or which key you're in while pinching, and then through relation you figure out what the harmonic'd note is by listening to it.
Quote by KTHXBAI
Yea but how do you know what pitch is A B C D E F or G?

You have to measure how far away from the bridge you're pinching.