There are also interline agreements that consist only of the exchange of cargo and not passengers. The list of interline partners is not necessarily important. For passing cheque services, BA requires that journeys be booked together. If this is the item, you need to use the service. If not, you won`t (even on BA). Experflyer has a free trial subscription, if you invest a little time, you can get the interline list yourself from there. As suggested in the USBT article above, you can do this yourself by searching for flights on EF or KVS that, in turn, are looking directly for a GDS, or by using the website of an online travel agency like Expedia. If the combination of BA and the other airline appears on a single route, there is an interline agreement to that effect. And when it comes to checking whether a route can be booked on a third-party site (with protected connections), then the answer is yes, unless you use Kiwi.com (the only book that gathers unprotected connections) .