In India, does a person's last name determine his religion?

In almost every country this is so.

If surname is Khan etc - Muslim

Taylor etc - Christian

Shukla etc- Hindu

Sometimes. If they have an Arabic name, they are probably Muslim (Ahmad, Yusuf, Ali, etc). If they have a European name they are probably Christian (D'Souza, DeMello, Russell, etc.). If they have a Parsee name they are probably Parsee (Ruzbih, Wadhia, Duruwalla, etc.). But it's not always obvious; sometimes people named Patel are Hindu, sometimes they are Jain. People named Singh are usually Sikhs. If they are named after a Hindu god they are most likely Hindu (Krishna, Radha, Rama, etc.) However, as someone pointed out, some Indians change their religion or have no religion at all.

90%

not necessarily. Besides, there are people there who change religions, but who don't change their name. What then?