Go to the website for the airline on which you are flying, and make sure you have access to the email you received confirming your booking. Click the link on the airline's website for checking your information. On the American Airlines website, this link reads My Trips/Check-In, as of 2015.
Enter the requested information. This generally includes the passenger's first and last name and the record locator. The record locator is a six-character code unique to the reservation and appears in the confirmation email.
If you don't have the record locator, contact the airline by telephone. Find the customer service number on the airline's website, and choose the menu option that allows you to inquire about existing reservations. As long as you know the passenger's first and last name, the departure and arrival city, and the travel times, the booking agent should be able to help you. The agent may ask some additional questions to confirm the passenger's identity.
Once you have entered the record locator online or confirmed identity with the booking agent on the telephone, you should be able to reconfirm the booking. Some airline websites allow you to change your seat choice and opt to pay for checked bags. Some airlines offer a discount for paying ahead of time to check your bags.