Justice Bhagwati in S.P. Gupta’s case pointed out the essentials of PIL, as under:

• There must be a legal wrong caused to a person or to a determinate class of person, on whom burden is imposed in violation of law or without legal authority.
• The wrong must arise from a violation of any constitutional or legal right.
• The wronged person (or determinate group of persons) must be unable to approach the court for relief by reason of – 
• Poverty, 
• Helplessness; or 
• Social or economic disability or 
• Socially or economically disadvantaged person.
• If the above conditions are satisfactory, then any member of the public can seek judicial relief for the above wrong.
• But the court should be anxious to ensure that the person initiated the proceeding to acting bonafide to get redress for a public grievance and not to pursue personal gains or from malicious motives.
• If the case is otherwise appropriate for PIL, then the court can act even on a letter addressed to it (SP Gupta Vs Union of India). 

Further Reading:

• Public Interest Litigation (PIL) in India
• Indian Polity Notes