Hypertension high blood pressure

How do doctors find out if I am suffering from hypertension high blood pressure?  Simple, they take their pressure monitoring instrument and look for my pressure readings.  The normal blood pressure of an individual, I am told is said to be 120/80.  The pressure readings are monitored on mmHg.  I was curious to know what my pressure reading was.  The doctor told me that it was 150/100 and I could be suffering from Hypertension.  He asked me about my past medical history and prescribed me some tablets.