I've read this question and its answers basically asking why putting out an engine fire is not left to the computer but a manual task.
One of the points mentioned is that there simply might be a sensor error or similar and the engine isn't actually on fire, in which case you obviously don't want the computer to needlessly shut down an engine.
My question however is, how do the pilots assess wether the engine is actually on fire or not?
All planes I've been on had their engines very close to the aircraft and I doubt they are easily if at all visible from the flight deck. I would assume a good indicator might be the loss of thrust, but that could also be detectable by a computer. Do the pilots have cameras they can activate to check on the engines, or do they call the cabin personal and ask them to take a peek, do they walk out and check or is it based on how loud the passengers scream in the back? I guess all of these work, but what is the common procedure?