How to tell if rash is chickenpox?

Google : The appearance of chicken pox is different at different points in the illness. I would suggest using Google to get pictures of chicken pox and compare them to what you see. Since most young people are vaccinated against the chicken pox, it is not commonly seen anymore.

Chicken pox: Rash of chicken pox consists of fluid filled blisters. They appear first on chest or abdomen and then spread to other parts of body. http://www.dermnetnz.org/topics/chickenpox/