A blood, urine, or saliva test can be done to measure your cortisol levels.
There are lots of lifestyle changes you can make to lower your stress and cortisol levels:
Björntorp, P., & Rosmond, R. (2000). Obesity and cortisol. Nutrition, 16(10), 924-936.
Chandola, T., Brunner, E., & Marmot, M. (2006). Chronic stress at work and the metabolic syndrome: prospective study. Bmj, 332(7540), 521-525.
Hofmann, S. G., Sawyer, A. T., Witt, A. A., & Oh, D. (2010). The effect of mindfulness-based therapy on anxiety and depression: A meta-analytic review. Journal of consulting and clinical psychology, 78(2), 169.
Grossman, P., Niemann, L., Schmidt, S., & Walach, H. (2004). Mindfulness-based stress reduction and health benefits: A meta-analysis. Journal of psychosomatic research, 57(1), 35-43.
Van Eck, M., Berkhof, H., Nicolson, N., & Sulon, J. (1996). The effects of perceived stress, traits, mood states, and stressful daily events on salivary cortisol. Psychosomatic medicine, 58(5), 447-458.
Torres, S. J., & Nowson, C. A. (2007). Relationship between stress, eating behavior, and obesity. Nutrition, 23(11-12), 887-894.