A reagent that can distinguish cis-cyclopenta-1,2-diol from the trans-isomer is:

Acetone reacts with 1,2-diols to form cyclic ketals (acetonides). This reaction is much more favorable for the cis-isomer because the two hydroxyl groups are on the same side of the ring, allowing them to easily link with the same acetone molecule. In the trans-isomer, the distance between the groups is too large for easy cyclic ketal formation.