If you have reserved a seat, or if it was automatically reserved for you by the airline, you can see this on your boarding pass. You will receive your boarding pass from the airline after completing your (online) check-in.
Before that time you can also sign in on the airline's website to check your seat number. Use your online check-in number (listed on your e-ticket) to log-in on the airline's website.
All Seats & Check-in questions Before that time you can also sign in on the airline's website to check your seat number. Use your online check-in number (listed on your e-ticket) to log-in on the airline's website.