The most practical way is to contact the registry office where you were registered or the hospital - they usually have the exact time in the original document. If that doesn't work out, you can use an astrology website (like astro.com) and enter the approximate time your mom remembers, like 9 in the morning, and see which rising sign makes the most sense with your personality, since the rising sign changes roughly every two hours. There's also the option of paying for a consultation with an astrologer who sometimes can figure it out from your face and demeanor, but that's more expensive and less reliable than going straight to the source.