Once the ticket is confirmed, you will receive a PDF that indicates your coach and seat number. Your seat/berth number is on the ticket. The conductor will check the ticket at your seat when the train leaves the platform.
First-class tickets do not show seat numbers. The seats are allocated and announced on the train chart 4 hours before the departure.
Once the seating chart is officially settled, you can see the printed chart near the door of the compartment, at the counter inside the station, or check your seat from your PNR status online: http://www.indianrail.gov.in/pnr_Enq.html