Are there tests to confirm the doctor's diagnosis of histoplasmosis?

Yes, but. Most of the time, certainty behind the diagnosis is unnecessary, because we don't usually treat it. Many ways to diagnose "histo", each of which has limitations. Culture of the bug from the body is one, blood/urine tests can suggest (but not confirm, or exclude) infection with histo, and lastly a simple history of your symptoms, the appearance of the xray (or ct) and where you live.