Voting is not mandatory in India. But to chose a proper Government at every level, citizens must exercise their rights to vote. Answering the second part, I must state that if a person votes, then his/her fingers will be marked to ink. Then everyone (including the workers of the political parties) will know whether you have votes or not.
However, I would advise you not to refrain from voting. If you are not satisfied with any of the candidates, you may exercise NOTA option.
Caution: Explosive. Handle with care.