BTR or Birth Time Rectification and is a long drawn out process. It is a trial and error method trying out various possible time details and check with the major past events in your life which is very tough. If the time and the major event times do match or/and if the characteristics of the horoscope matches with your self, one can presume the time of birth is very accurate.
Needless to say BTR can only be done using consultations in person and it will be very expensive too.