How does the L-glyceraldehyde differ from the D-glyceraldehyde?

D and L are easily determined by looking at the hydroxy group that is farthest away from the aldehyde group. If it is on the right, it is the D enantiomer. If it is on the left, it is the L enantiomer. Each enantiomer will rotate plane polarized light in opposite directions. This can be determined using a polarimeter.