Hair loss is not a guessing game.

Guessing is easiest for men's hair loss

If you had to "guess" the reason for someone's hair loss, you'd likely guess correctly if the patient was male and you chose male balding (androgenetic alopecia). By far that's the most common cause of balding in men.  Dozens of other causes are possible, but they are not common. 

Hair loss in women is more complex.

For women, there's a very good chance you'd be wrong if you guessed androgenetic alopecia as the sole cause. Female hair loss is far more complex - and hair shedding issues and even scarring conditions are not uncommon. Hair loss of course, is not a guessing game and a diagnosis can usually be made by the patient's history, examining the scalp and sometimes examining blood test results or (rarely) performing a scalp biopsy.