How do you tell if finasteride is slowing or stopping the hair loss?

It is sometimes hard when you have slowing or stopping because it compares it against what would have happened had you not taken it.  Take good photographs.  if you have a doctor with the HAIRCHECK  system (https://baldingblog.com/haircheck-test-how-it-is-done-video/) which will give you a number for hair loss and hair gain if your drugs are reversing it.