Here are all the supported radio protocols listed and where to connect them : Radio Control Systems
If you want to know which protocol you are actually using, it is probably the protocol that your receiver is set to.
Also if you know what protocol you expect, but aren’t sure if ArduPilot detects it correctly, you can limit the protocols it is looking for with the RC_PROTOCOL parameter.
Whilst I agree with you, the process of elimination would ascertain the answer, I would have thought there would be something in the log to state which protocol is being received. I cannot find any reference to this so I guess its not to be.
Go to Mission Planner “Dataflash Logs” tab, click “Convert .Bin to .Log” and select your logging file.
Then open that file with a text editor and search for “RC Protocol”, for me it looks like this: