In my case, I had a rectal swab performed by a parasitologist, so he basically took some mucus out of my butt and then looked at it in a microscope. Appetizing!
For me, I really have no idea if Finasteride did anything for my hair but I doubt it. I only took it for two weeks several years ago, but I still have a decent amount of hair, so maybe I got sort of lucky with only "go kind of bald" genes?