Most airlines allow their passengers to check online for a flight online. That means you need to visit the airline side and check-in for a flight there.
Online check-in requires you to have with you:
If the airline does not allow you to check-in at the moment you want to be notified on the page:
If you fly a RyanAir airline and do not have a European Union passport, pay attention. When you complete your online flight application, you will receive an e-mail boarding ticket that you will need to print. If you want to check before the deadline, it is necessary to make an additional payment. the company calls this a priority boarding and seat purchase.
On the other hand, Wizz Air allows you to have your billing tickets stored on your phone, but there are destinations that cannot be logged in online at all. In this case, check at the airport.