Going back to the original question, which was asking about diagnosis as opposed to actions.
I think it’s a really good one. We all know (or should know) the memory drills for unreliable airspeed in our various types. When to apply them is more of a problem area, as the aircraft will almost certainly be deviating from its previous flightpath when you do, with varying levels of excitement depending on how busy the environment is.
It’s all very well sitting in the the simulator doing a UA exercise but there are too many levels of unreality here, IMHO. First, you know it’s coming and second, there are only so many ways to get a sim to behave.
In real life, there are many situations where the pitch/power relationship can be well away from normal but all the aircraft systems are functioning correctly. Some are dynamic and some can last a considerable time. If you encounter horizontal shear or temperature gradients, or climb/descend through a vertical shear, you will get pitch/power/airspeed combinations very reminiscent of a UA scenario.
How to tell the difference? Well, if you’ve got a conflict of opinion between airspeed sources, then it’s a genuine problem. If you haven’t, it’s still possible that they have all been affected at around the same time. When I had ice crystal icing a few years back, it happened to both engines within seconds of each other, which I suppose is not that incredible seeing as they’re virtually identical.
If the attitude is not what you expect, it could be from external forces or driven by false data interacting with the auto flight systems. FBW and non-FBW aircraft behave differently AP-out as well. I’m not at all surprised that it takes some time to register that not all is right as the symptoms of flying through changing wind and/or temperature are very similar to those you might get with certain types of UA.
In summary, I don’t think it is that obvious initially in many cases that you have a UA problem, given that during almost every flight there are instances of UA-like symptoms which are simply due to environmental changes.