I don't think measuring serum cortisol levels would be a great method of testing for stress. It is expensive, painful, and the results are difficult to interpret. Salivary levels might be better, but this would also be expensive.
To actually determine the effect of stress on cortisol levels, you would need to control for variables including (but not limited to): age, gender, sex steroid levels, pregnancy, lactation, smoking, coffee and alcohol use, diet, level of exercise, genetic factors and habituation to repeated psychosocial stress exposures.
An easier and less costly method would be before and after blood pressure and heart rate measurements.
 Why do we respond so differently? Reviewing determinants of human salivary cortisol responses to challenge? Psychoneuroendocrinology Volume 34, Issue 1, January 2009, Pages 2-18