It usually doesn’t require any medical test to diagnose chickenpox. We can be pretty sure it is chickenpox if it has symptoms of mild fever followed by an itchy rash, with blisters and scabs.
This chickenpox rash is usually distinctive enough to distinguish from other rashes. Sometime it is more confused with other conditions that affect the skin, such as insect bites or scabies.
If you're still uncertain about what is causing the symptoms, chickenpox can be confirmed with laboratory tests, including blood tests or a culture of lesion samples.