If you develop red spots on your trunk and face that itch, you may suspect that you have chickenpox, especially if you have never had chickenpox before and have not been vaccinated against varicella. If you have also recently come into contact with someone who has chickenpox or shingles, there is a good chance that you have chickenpox.
In most cases, a doctor can diagnose chickenpox based on the appearance of the rash. If necessary, the fluid from a blister can be tested for the presence of varicella virus or a blood sample can be taken to test for antibodies against the virus.