According to a ruling by the Bombay High Court in 2013, an Indian passport cannot prove citizenship. Also, in 2019, the Government of India claimed that 'common documents' were enough to prove citizenship.
What are the documents or paperwork which can prove someone is an Indian citizen?
Reference: https://theleaflet.in/indian-citizenship-law-a-mess-proving-citizenship-even-messier/
What are the documents or paperwork which can prove someone is an Indian citizen?
As is often the case, it depends on the context: who is seeking to prove the person's Indian citizenship? For what purpose? How did the person become an Indian citizen?
For example, if the person became Indian by naturalization, a naturalization certificate would usually be necessary. If a person became Indian by birth then evidence establishing the relevant facts will be needed, and this evidence will typically include a birth certificate.
We can see that a passport isn't necessary to prove citizenship because a first-time passport applicant needs to prove citizenship in the application. Whatever documents are acceptable for this must therefore constitute proof of citizenship.
But there is always the possibility that an Indian citizen in possession of any document, including a passport, does something that causes the automatic loss of Indian nationality, such as naturalizing in a foreign country. At that point, the former Indian citizen will be in possession of documents that appear to show Indian citizenship without actually being an Indian citizen. Therefore, we can establish that no document or set of documents can be conclusive proof of Indian citizenship.
Depending on the context, the person who needs to be convinced of a person's Indian citizenship may or may not need to take into account the likelihood that the person has somehow done something that would cause loss of citizenship. An airline agent boarding a passenger for an international flight to India will be satisfied by the passport. The immigration officer processing the passenger on arrival might however make a brief inquiry into possible expatriating acts. A judge in a court case may go as far as to undertake an intensive legal-factual analysis in which documents play a limited role, if any.
Thus, the answer to your question
What is the sure shot way of proving citizenship of India?
is that there is none. There is always the possibility that a good-faith attempt to prove the citizenship of an Indian citizen will fail. There is always the possibility of a noncitizen having documents that were issued legitimately and purport to show that the person is an Indian citizen.
In the 2013 case, it appears that the passports were issued in error. The court did not rule that passports cannot be used to prove Indian citizenship but that it "may not be enough to prove you are an Indian citizen if you were born after July 1, 1987" (quoting the Times of India here, not the court).
The Indian citizenship law states that (emphasis mine)
Indian citizenship can be acquired by birth, descent, registration and naturalization. The conditions and procedure for acquisition of Indian citizenship as per the provision of the Citizenship Act, 1955 are given below:
(1) By Birth (Section 3)
A person born in India on or after 26th January 1950 but before 1st July, 1987 is citizen of India by birth irrespective of the nationality of his parents.
A person born in India on or after 1st July,1987 but before 3rd December, 2004 is considered citizen of India by birth if either of his parents is a citizen of India at the time of his birth.
A person born in India on or after 3rd December, 2004 is considered citizen of India by birth if both the parents are citizens of India or one of the parents is a citizen of India and the other is not an illegal migrant at the time of his birth.
An 'illegal migrant' as defined in section 2(1)(b) of the Act is a foreigner who entered India.
(i) without a valid passport or other prescribed travel documents : or
(ii) with a valid passport or other prescribed travel documents but remains in India beyond the permitted period of time.
(2) By Descent (Section 4)
A person born outside India on or after 26th January 1950 but before 10th December 1992 is a citizen of India by descent, if his father was a citizen of India by birth at the time of his birth. In case the father was a citizen of India by descent only, that person shall not be a citizen of India, unless his birth is registered at an Indian Consulate within one year from the date of birth or with the permission of the Central Government, after the expiry of the said period.
A person born outside India on or after 10th December 1992 but before 3rd December, 2004, is considered as a citizen of India if either of his parents was a citizen of India by birth at the time of his birth. In case either of the parents was a citizen of India by descent, that person shall not be a citizen of India, unless his birth is registered at an Indian Consulate within one year from the date of birth or with the permission of the Central Government, after the expiry of the said period.
A person born outside India on or after 3rd Decmber, 2004 shall not be a citizen of India, unless the parents declare that the minor does not hold passport of another country and his birth is registered at an Indian consulate within one year of the date of birth or with the permission of the Central Government, after the expiry of the said period.
Thus the "paper" necessary to prove citizenship varies by your date and place of birth.
If you were born in India before July-1-1987, a birth certificate from relevant authority is enough.
If you were born in India after that date but before Dec-3-2004, you need to have your birth certificate and birth certificate of either parent (assuming they were born in earlier time frame).
If born after that time, you need birth certificates for yourself and your parents. If the birth certificates are not enough to prove parents' citizenship, you need birth certificates of your grandparent(s) too.
Note that this answer only touches upon the "citizenship by birth" aspects. Other aspects (naturalization, renounciation) have been already explained well by phoog in his answer.
