> How can entangled system be differentiated from a nonentangled system?
The canonical answer to your question is Bell's inequality: https://en.wikipedia.org/wiki/Bell's_theorem. But TL;DR: the distinction only shows up in the statistics of repeated experiments. There is _no way_ to distinguish them in single-fire experiments. Entanglement is defined in terms of "odd" statistics.
In repeated measurements of related properties (e.g. spin along varying angle), entangled systems show more correlation than it should be possible classically.
The canonical answer to your question is Bell's inequality: https://en.wikipedia.org/wiki/Bell's_theorem. But TL;DR: the distinction only shows up in the statistics of repeated experiments. There is _no way_ to distinguish them in single-fire experiments. Entanglement is defined in terms of "odd" statistics.
In repeated measurements of related properties (e.g. spin along varying angle), entangled systems show more correlation than it should be possible classically.