As proposed in the USBT article, you can do this yourself by searching for flights on EF or KVS that, in turn, go directly through a GDS, or using the website of an online travel agency like Expedia. If the combination of BA and the other airline appears on one line, there is an Interline agreement for this. Interline chords are turning points. For example, American Airlines may be able to issue the ticket on an American United route, but United may not be able to issue on the same route. A single interline agreement is called a one-sided interline. Airlines may also agree to a bilateral interline agreement in which each airline can issue the ticket to the other carrier. Even the best business travel professionals will make the GDS simple for the necessary trip according to the authorities, instead of working backwards from a list. If the airline combination is possible on a transit ticket because there is an Interline agreement, it is displayed on an incredibly long list, which can be sorted in different ways. There are also interline agreements that exist only for the exchange of freight and not for passengers. Experflyer has a free test subscription, if you invest a little time, you can get the Interline list yourself from there. If there is no interline ticketing agreement, two separate tickets must be issued and passengers must pick up their luggage and take it to the connecting company for check-in.
Interline routes such as this one are more risky for travellers, as the second airline may not be aware of inbound flight delays or problems and is less likely to authorize a toll-free change of booking in the event of a loss of the route. There may also be a problem if the baggage is lost and the traveller wishes to be sent to them later. The interline, also known as interline and interline ticketing, is a voluntary commercial agreement between different airlines to treat passengers travelling on routes requiring several flights with several airlines.  Such agreements allow passengers to switch from one flight with one airline to another airline flight without having to pick up their baggage or re-register it. Airlines can also promise a free booking change if the service is interrupted due to a delay. The only places I know the information is available are Expert Flier and KVS Availability Tool. EF is a subscription and KVS is only Windows (my personal laptop is a MAC) and I don`t have a subscription. If someone does, they could provide the list. Another way is to ask on Flyertalk, because someone will have the list there – flyertalk.com/forum/…
But if an airline is in IATA, there is a chance that BA will have an Interline agreement. But many airlines are not in IATA – especially low-cost airlines who think membership is one of the costs they can avoid. Interline E-Ticketing bilateral agreements (BIETA/(passengers only) allow you to get up-to-date information on 10,600 electronic interconnection agreements to update your databases directly. It is available as an Excel file. Updated daily and accessible via secure access to the ecstasy or distributed via FTP. And if you check that a route can be booked as an itinerary (with protected connections) on a third-party site, then the answer is yes, unless you use Kiwi.com (the only book that paves unprotected connections). You shouldn`t just buy from a third party online because of other shonkiness levels, but, kiwi aside, third parties use a GDS for flight search and GDS will only return schedules with protected interlined connections. If one of them looks good, take the details of a serious full service travel agency and see if they can book it. A full list of Interline partners for an airline as important as BA will not be available publicly on the airline`s website (z.B BA).
The list includes fierce rivals like Virgin Atlantic and BA will not advertise.