A doctor usually knows your child has roseola because of the telltale symptoms: high fever followed by rash. Usually, no lab tests are needed.
Since it’s caused by a virus, antibiotics won’t help cure it. So, your child’s doctor will likely just treat the symptoms.
For high fever, your child may get acetaminophen or ibuprofen. If more symptoms or your child becomes very sick, they may have blood or urine tests.