Always depends on the airport I am flying in/out of, airline I am flying with, etc.
, you can get the registration information for all flights (including military, executive charter, cargo, etc.) via the FIS (airport information system). After selecting the language you have to select OPS inbound or OPS outbound. There you can get the registration of all planes operating in and out of FRA
, e.g. they have the registration of the plane printed on the cabin wall in big black letters on the right hand side when you enter the plane via the front door.
Sometimes I just ask the check-in agent for it as they can see it in their system as well.
Then there is, of course, also the possibility to look out of the window. Usually I try to combine most of the methods above to make absolutely sure that I get the right registration.