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

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.

Dermatologist looks at where the changes are occurring. Is it all over or is it localized? And the dermatologist looks at changes in follicle size as well as where on the scalp there is a positive pull test. Taken together one can tell whether there is AGA or not in most cases. Early AGA is tricky - and the experience of the physician will be important to differentiate from hairline maturation and normal shedding. 

Early balding is best determined with an instrument called HAIRCHECK which measures the loss of hair bulk in different parts of the scalp and shows hair loss before you can see it with the naked eye

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!