An exciting property of chiral compounds is their ability to rotate light.
They are able to rotate plane-polarised light either clockwise or anti-clockwise. The direction of rotation is defined by which enantiomer the light is being shined through. The convention for the names of the c-wise and anti-c-wise are (+) and (-) respectively.
The direction of the rotation cannot be guessed with the knowledge of whether the compound is R or L.

The usual way for telling the rotation is using a polarimeter.
The way that it works is: Light from a (usually single wavelength) point source, is passed through a Nicol prism becoming polarised. This light passes through a sample of your chiral compound, and is rotated. You can then work out how much the light has been rotated by using an analyser.

The amount that the light is rotated depends on different substances, but also the purity of a mixture.
If a mixture is racemic, then it contains equal amounts of both L and R enantiomers. If this is the case then the two ways of rotation will cancel each other out. If a sample is pure, then you will get the maximum amount of rotation. If, however, you get some of the other enantiomer in the sample then the overall rotation will not be as great.