Visit website of the concerned public authority and scrutinize RTI icon. For state information, also visit official portal of state govt and information commission for these details. If you do not get details of PIO, visit nearest office of relevant public authority and try to get address of PIO. Name is not important, as officers do get changed.
If you do not still succeed, please address your application to PIO C/o. Head of the dept/office to which information is required. If you want information of a district, you can address application to PIO of District Collectorate.
If the envelope is not accepted or returned by post office, send the original application with photocopy of envelope to the PIO of Office of Chief Secretary at State Capital or connected ministry at Delhi, with a request to forward the enclosed application to the concerned PIO. A reference to india.gov.in website will be useful.
Alternatively, you can also file RTI application for the same information with PIO of Office of Chief Secretary at State Capital or connected ministry at Delhi, and PIO will redirect the application to correct PIO within 5 days of receipt by him with intimation to you.
Simultaneously, file a complaint with Central Information Commission/State Information Commission for difficulties in locating PIO of a particular dept/office.
You can take help of local/state-level NGOs or RTI helpline phone numbers for locating correct PIO, who would be holding information required by you.
Most of Central Govt public authorities have placed details of PIO etc on their websites. However, states are yet to follow. Some state information commissions do not have details of PIO/FAA on their website!