Comments from Sikh Scholars about who is a Sikh

Retention of the Five K's, Paper by Dr. Jarnail Singh
The baptism of a Sikh has to be voluntary act, ie., the person has to offer himself, and the ceremony can be performed by five individuals already baptized. The baptism in Sikhism is a further stage and a person could stay in the Sikh faith for a long time without being baptized. While wearing of the five K's is not obligatory for non-baptized Sikhs, they do start wearing these symbols long before being baptized. In a sense it is the evolution of the individual towards the final goal, keeping of unshorn hair is usually the start of the process. Hence every Sikh, baptized or not, has the right to wear these symbols.

Sikhism: A Complete Introduction by Dr. H.S. Singha and Satwant Kaur, Hemkunt Press, Delhi
The basic transformation in the concept of an ideal Sikh from Guru Nanak to Guru Gobind Singh is the transformation from a Sikh to a Singh. A Singh is a person who has reached the goal; who has realised the Self and whose own self is therfore no more; whose ego and little personality are shed off and destroyed; and who thus has no proper particular home and so is designated by the generic term Singh so long as he is active in the social and political context of the Sikh way of life. The term Sikh on the other hand implies a person who is a learner, who is set on the path of spiritual perfection and self-realization but who has not yet fully realised the Self, who has not yet found the Truth in entirety. When a Sikh reaches the final goal and ceases to act with the fulcrum of his ego, he becomes a Singh, the perfected one.

The Question of Sikh Identity, Paper by Gurutej Singh
As long as Sikhs refuse to practice daily the lifestyle set down by the ten Gurus, there will always be ignorance, in-fighting, slandering, and lack of co-operation and communication between Sikhs as well as between Sikhs and non-Sikhs. Politics will continue to take precedent over prayer, because people forget how to pray or become so comfortable with material possessions that prayer becomes redundant. It is only through daily Sadhana, doing path, living together as community, refraining from alcohol and drugs, singing Gurbani Kirtan, chanting God's name, and living in God-given form, that a Sikh becomes a Sikh. To discuss who's a Sikh and who's not a Sikh beyond this is meaningless.