How can we know that a baby can hear and see at birth?

Well, close observation of doctors and parents plays a very important rote in this.  Although there are specific tests to look for intact hearing and visual pathways parents can assess hearing by nothing baby's reaction to sound in a claim and quite room.

If there is sudden loud sound the baby reacts to it by showing a gesture of raising his arms and flexing the legs or starts crying as if he did not like the sound.  This can be assessed from birth.  As far as vision is concerned babies tend to squint on focusing even the mother's face and vision improves gradually.  They star focusing the mother's face or objects close to their face by the first month's face or objects close to their face by the first month. These can, however be checked and assessed even at early stages.