Question

How to find which compound is dextro and leave rotary from its structure?

Answer 1

An old problemoe....

Explanation:

As I understand it, when we use terms such as `"dextrorotatory"` or `"laevorotatory"` we speak of a macroscopic experimental phenomenon: i.e. the direction a solution of the stereoisomer will rotate plane-polarized light. THIS HAS NO NECESSARY RELATIONSHIP WITH THE ABSOLUTE, GEOMETRIC CONFIGURATION of the stereoisomer. That is we can say that the enantiomer will polarize light in the opposite direction; we cannot necessarily say which direction a given enantiomer will polarize the light.

The absolute configuration of a stereoisomer can only be assessed by a specialized X-ray experiment, or maybe a circular dichroism experiment (which I have never done, so I cannot be much use to you), which is a technique that is a bit more accessible.

Anyway, there may be more expert opinion available here.

As to how optical isomers are separated, see here.