To identify any enantiomer or diastereomer by circular dichroism, you need cd spectra of one of the enantiomer or diastereomer at known concentration.

for example, (R,S) isomer and (S,R) isomer give opposite spectra( positive or negative cotton effect) and for the same compound (S,S) or (R,R) will also give opposite spectra. CD spectra for (R,S) and (S,S) may be same but intensity must be different at same concentration.

so if one have CD spectra of one or two isomer, other conformation will be known by comparison.