How can two optical isomers be distinguished?
By their rotation of plane-polarized light.
Given quantities of each isomer, one isomer, one so-called optical antipode, will rotate plane-polarized in one direction by some angle,
Admittedly, these are difficult experiments to do, and often one isomer is not as optically pure as the other isomer. Isomers that derive from natural sources usually have high optical purity (of course this purity relates to the one isomer only and not its antipode, which may be unavailable).