How can a dermatologist determine if a patient has has mpb at the very earliest stages?

Hair loss needs an in-person evaluation. There can be many reasons for hair loss. I recommend getting a formal evaluation with a hair loss expert to go over your condition and advise you on best treatment options. There are great non-invasive options like prp/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer

Ha!  There are two ways to answer this question.
The easy answer is for a physician to look for tiny "miniaturized" in a pattern consistent with male pattern hair loss.  90+% of the time the diagnosis can be made by just looking at the patient and discussing their family history.  A microscopic device called a dermatoscope is very very useful and a serious hair loss specialist will use one for an exam in a patient like this.
The difficult answer is that it depends on the patient.  Some patients have finer hair, unusual balding patterns, or skin conditions.  The amount of shedding is not usually an indication but it can complicate a diagnosis.  The trick is to go to someone who specializes in hair alone and has several years of experience.  They would be able to pick up on the subtle cues and have the experience to give each patient the right diagnosis.
Good luck!

Your best approach is to have a full history and exam with an expert in hair loss. Classic patterns are often seen in the spectrum of hair loss, and other tests may help if needed.