It shouldn't really be all that difficult to determine
AC-130s have very elaborate and capable night imaging systems. I've seen video footage from older AC-130s and it was pretty impressive. It would really surprise me if no one had a video recorder on board that plane, similar to all those bomb cameras we saw from the Gulf War. It would only make sense - a video recording would clearly show whether there was ground fire. That might mean compromising some of the system's detection capabilities but it would seem worth it in this case to get to the truth.