Measure the left half of the peak width at 10% peak height (A) and then measure the right half of the peak width at 10% peak height (B) (see figure below).  The ratio of B over A is the asymmetry factor.  A value of 1.0 is perfect symmetry and 0.9 – 1.2 is acceptable.  Value less than 1 is for a fronting peak and greater than 1 is for a tailing peak.

Values greater than 2 are generally unacceptable.